How to get Adview below Viewpager in CoordinatorLayout

You can add ViewPager and AdView inside RelativeLayout and specify android:layout_above="@+id/adView" property to ViewPager so that adView doesn't block the ViewPager contents.

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:ads="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.candyx.testapp.Activity">

    <android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:id="@+id/main_content"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_above="@+id/adView">

        <android.support.design.widget.AppBarLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="@drawable/main_button_background">

            <android.support.v7.widget.Toolbar
                xmlns:android="http://schemas.android.com/apk/res/android"
                android:id="@+id/toolbar"
                android:layout_width="match_parent"
                android:layout_height="?attr/actionBarSize"
                app:layout_scrollFlags="scroll|enterAlways"/>

            <android.support.design.widget.TabLayout
                android:id="@+id/tabs"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                app:tabMode="scrollable"
                app:tabGravity="fill"
                app:tabIndicatorColor="@color/colorTabIndicator"
                app:tabIndicatorHeight="3dp"/>

        </android.support.design.widget.AppBarLayout>

        <android.support.v4.view.ViewPager
            android:id="@+id/viewPager"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:layout_behavior="@string/appbar_scrolling_view_behavior"/>

    </android.support.design.widget.CoordinatorLayout>

    <com.google.android.gms.ads.AdView
        android:id="@id/adView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        ads:adSize="BANNER"
        ads:adUnitId="@string/banner_ad_unit_id"
        android:layout_gravity="center|bottom"
        android:layout_alignParentBottom="true"/>

</RelativeLayout>

I tried the answer by @AvishekV but that didn't solve my problem. But I took THE IDEA of putting CoordinatorLayout and Adview inside a parent layout which in my case was ConstraintLayout. Below is the structure

<android.support.constraint.ConstraintLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:ads="http://schemas.android.com/apk/res-auto"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

   <android.support.design.widget.CoordinatorLayout
       android:id="@+id/content"
       android:layout_width="0dp"
       android:layout_height="0dp"
       android:fitsSystemWindows="true"
       app:layout_constraintBottom_toTopOf="@+id/adView"
       app:layout_constraintEnd_toEndOf="parent"
       app:layout_constraintStart_toStartOf="parent"
       app:layout_constraintTop_toTopOf="parent">

        <android.support.design.widget.AppBarLayout>
           //CollapsingToolbarLayout
           //ConstraintLayout, if you need to put anything inside collpasingtoolbar
           //Toolbar
           //TabLayout
        </android.support.design.widget.AppBarLayout>

        <android.support.v4.view.ViewPager
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior" />
</android.support.design.widget.CoordinatorLayout>

<com.google.android.gms.ads.AdView
    android:id="@+id/adView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    ads:adSize="SMART_BANNER"
    ads:adUnitId="your adunit id"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintHorizontal_bias="1.0"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintStart_toStartOf="parent" />

</android.support.constraint.ConstraintLayout>

This worked for me : just put inside the ViewPager the app:layout_behavior :

<android.support.v4.view.ViewPager
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"

Put ViewPager and Adview inside a another LinearLayout as below:

    <LinearLayout 
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">
     <android.support.v4.view.ViewPager
            android:id="@+id/viewPager"
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_weight="1"
            app:layout_behavior="@string/appbar_scrolling_view_behavior"/>

        <com.google.android.gms.ads.AdView
            xmlns:ads="http://schemas.android.com/apk/res-auto"
            android:id="@+id/adView"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            ads:adSize="BANNER"
            ads:adUnitId="@string/banner_ad_unit_id"
            android:layout_gravity="center|bottom"
            />
    </LinearLayout>