Fixes#2188
We get a call to stop the audio channel from a timer/IRQ context, so can't
safely remove the IRQ handler for the AudioBufferManager. The SDK will panic.
Because the IRQ handler will be a noop if it's not uninstalled, we will
instead just leave our shared handler in place and let it do nothing.
Use a common BluetoothLock RAII in BluetoothAudio to clen up the code and
automatically lock BT for the AVRCP button methods.
Earle F. Philhower, III
bf385490d3