Fix some lockup caused by handles not being initialized to < 0...

Fix some lockup caused by handles not being initialized to < 0...

...by default where they would be interpreted as valid but not actually
be which would cause calls to buffering while it was not initialized.

Add BUFFER_EVENT_BUFFER_RESET to inform users of buffering that the
buffer is being reinitialized. Basically, this wraps all the
functionality being provided by three events (...START_PLAYBACK,
RECORDING_EVENT_START, RECORDING_EVENT_STOP) into one for radioart.c,
the only user of those events (perhaps remove them?) and closes some
loopholes.

Change-Id: I99ec46b9b5fb4e36605db5944c60ed986163db3a