fdo#60486 Fix auto-enabling bluetooth and improve bluetooth handling.

We should only enable bluetooth with explicit approval of the user, see:
http://developer.android.com/reference/android/bluetooth/BluetoothAdapter.html#enable%28%29

We now also display an appropriate message if bluetooth is disabled.

Change-Id: Ic3a07c9ad0806a60ac7c7e609a30add7af18916f