Add PHY update function for Android Bluetooth LE

Android API 26 onwards supports setting the available PHY, i.e. 1Mbps, 2Mbps or LE Coded. There is an API function and callback for changing this as shown on https://developer.android.com/reference/android/bluetooth/BluetoothGatt.html#setPreferredPhy(int,%20int,%20int) which would be useful to add to Qt. As BLE connections can be established using 1Mbps and LE coded 500Kb and LE coded 125Kb and these are not compatible with one another it would be useful to set this before attempting to connect (and would be useful for other platforms too, e.g. iOS and Linux)