Have audio service clean up new receivers in crashing processes.

The new media button receiver with only a pending intent (no
component name) could be left hanging if the process that
registered it went away.  These semantically need to be tied
to the calling process's lifetime; we now clean them up when
the calling process goes away.

Also added some additional cleanup of media button receivers
when packages change (updated, cleared).

And on top of that, a new "media" command for doing media
things.  Currently lets you send media keys and monitor
remote display data.

Oh and finally added a new BaseCommand base class for
implementing these command line utilities.

Change-Id: Iba1d56f10bab1eec4a94a7bb1d1c2ae614c8bcf5