Clean up role of component name in media button event receiver

A public API was added to AudioManager to register a media button
 event receiver whithout having to supply a ComponentName.
This CL updates the comments that specified that the now optional
 ComponentName parameter can be null, and never persist the button
 event receiver is there is no ComponentName to persist.
Also unregistering the media button receiver makes no use of
 the ComponentName anymore.

Change-Id: Idcfed1e2a85feb6fa178ca7797427f238705957c