Toolbar - Switching from drawer to back button with only one Activity

Put this code into onCreate() of your Activity. Works well for me. Even using compileSdk 23 and higher.

    drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    final Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    if(toolbar != null) {
        toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        toggle.syncState();
        drawer.setDrawerListener(toggle);
        getSupportFragmentManager().addOnBackStackChangedListener(new FragmentManager.OnBackStackChangedListener() {
            @Override
            public void onBackStackChanged() {
                if (getSupportFragmentManager().getBackStackEntryCount() > 0) {
                    getSupportActionBar().setDisplayHomeAsUpEnabled(true); // show back button
                    toolbar.setNavigationOnClickListener(new View.OnClickListener() {
                        @Override
                        public void onClick(View v) {
                            onBackPressed();
                        }
                    });
                } else {
                    //show hamburger
                    getSupportActionBar().setDisplayHomeAsUpEnabled(false);
                    toggle.syncState();
                    toolbar.setNavigationOnClickListener(new View.OnClickListener() {
                        @Override
                        public void onClick(View v) {
                            drawer.openDrawer(GravityCompat.START);
                        }
                    });
                }
            }
        });

That's probably not what you would like to hear, but even from a conceptual point of view I would go for a new activity rather than a fragment.

Your main activity is strictly linked to the drawer, so loading a new fragment without any access to the drawer makes no sense to me (but feel free wait for other answers if you think so). A new activity would solve both problems, since it would have no drawer and could be a child of the main one.

Your side question looks spot on also. A "Add New" activity could nicely fit into the "full-screen dialog" visual pattern from the guidelines. See:

http://www.google.com/design/spec/components/dialogs.html#dialogs-full-screen-dialogs

This pattern has a "save", positive button on top-right, and a X. Conceptually, the X button is to cancel/abort a process, rather than navigating up some backstack. It means you are dismissing something without letting any action happen. This fits well for what you want to do.

From a design point of view, it's easily made by a new Activity, that can stay on top of others. Also, if the point of fragments is basically being able to represent two or more at once in tablets and bigger screen - again - I wouldn't be so happy with an old fragment on my left and an "Add New" fragment on the right.

Rather - on tablets - I would go for a floating dialog, as suggested by the guidelines.

http://www.google.com/design/spec/components/dialogs.html#dialogs-confirmation-dialogs

So full-screen activity with a X button for phones, and floating dialog (with buttons at the bottom) for tablets. This, to me, is the most guidelines-coherent approach.

---

I recommend reading the whole link. On the difference between <- and X,

The X differs from an Up arrow, which is used when the view’s state is constantly being saved or when apps have draft or autosave capabilities. For example, an Up arrow is used in Settings because all changes are committed immediately.

And also

Touching the X in this Settings example will discard all changes. Changes will be saved only upon touching Save.

It should work even for latest API 24.

In your activity onCreate() do this:

final Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
final DrawerLayout drawer = (DrawerLayout) view.findViewById(R.id.drawer_layout);
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);

final ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawer, toolbar, 
    R.string.navigation_drawer_open, R.string.navigation_drawer_close);
drawer.addDrawerListener(toggle);
toggle.syncState();

final View.OnClickListener originalToolbarListener = toggle.getToolbarNavigationClickListener();

getSupportFragmentManager().addOnBackStackChangedListener(new FragmentManager.OnBackStackChangedListener() {
    @Override
    public void onBackStackChanged() {
        if (getSupportFragmentManager().getBackStackEntryCount() > 0) {
            toggle.setDrawerIndicatorEnabled(false);
            toggle.setToolbarNavigationClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    getSupportFragmentManager().popBackStack();
                }
            });
        } else {
            toggle.setDrawerIndicatorEnabled(true);
            toggle.setToolbarNavigationClickListener(originalToolbarListener);
        }
    }
});