[iOS][BLE] Get negotiate MTU

iOS kicks off an MTU exchange automatically upon connection.

Devices running iOS < 10 will request an MTU size of 158. Newer devices running iOS 10 will request an MTU size of 185.

Assuming the device you are connected to supports these sizes, that is what you should see.

You should be able to determine the max payload size negotiated by looking at the maximumUpdateValueLength property of the CBCentral. (Note this will be 3 bytes less than the ATT MTU since that's the overhead for a ATT notification/indication)