Fix IllegalStateException when A2DP connects before boot completes.

Under some circumstances, the broadcast intent
BluetoothA2dp.ACTION_CONNECTION_STATE_CHANGED can be sent before
the system is ready, triggering an IllegalStateException,
"Cannot broadcast before boot completed" and runtime crash.

Fixed the race condition by adding the
Intent.FLAG_RECEIVER_REGISTERED_ONLY_BEFORE_BOOT flag
to the broadcast intent. All system receivers of this intent
are registered with registerReceiver() rather than as components in
the manifest, so nothing should change in the event that the A2DP
connection state change is broadcast before boot completion.

Any apps that define a receiver for either
BluetoothA2dp.ACTION_CONNECTION_STATE_CHANGED or
BluetoothA2dp.ACTION_PLAYING_STATE_CHANGED in their manifest
will not receive these broadcasts in the case where they are sent
before the system has finished booting. Normally, user applications
should not care about these events anyway and should let the audio
manager take care of audio routing on their behalf.

In the event that an app does care about A2DP state changes, it
should either register for these intents with registerReceiver(),
or it can add an intent filter for ACTION_BOOT_COMPLETED and
test the state of Bluetooth and A2DP at that time. Apps which
care about the state of the Bluetooth adapter should do this also,
because BluetoothAdapter.ACTION_STATE_CHANGED is already being sent
with FLAG_RECEIVER_REGISTERED_ONLY_BEFORE_BOOT.

Bug: 4982088
Change-Id: I10e55713f9d07d6dc88b4480b45b1aeb3aaf170b