No longer watching headset connect events

Now using AudioManager's methods to detect presence of headset -
isBluetoothA2dpOn() and isWiredHeadsetOn(). These are marked as
deprecated in the Android API documents, but is suggested for use by
Google's own docs
(http://developer.android.com/training/managing-audio/audio-output.html).

Manifest modified to increase the Android version required from 4 to 5,
and add the permission MODIFY_AUDIO_SETTINGS which is apparently
required for isWiredHeadsetOn() to work.