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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.