Fix logic for storing MediaButtonReceiver

Currently, MediaButtonReceiverHolder stores the
ComponentName.toString() value and tries to unflatten it with
ComponentName.unflattenFromString(String).
This causes restoring a stored MediaButtonReceiver to fail.

This CL changes it MediaButtonRecieverHolder store
ComponentName.flattenToString() value instead.

Bug: 169246173
Test: 1. Play music on YT Music.
      2. Reboot phone.
      3. Send media key event (PLAY)

Change-Id: I33449c0affd7f79e5988e7709cbbe45218766bf8