android_audioPlayer_metadata_* APIs don't exist on non-Android.
handlers.c needs a definition for NULL.
handler_AudioPlayer_gain isn't present on non-Android.
Fix typo in declaration of ThreadPool_add.
Fix warning: unused variable len.

Change-Id: Ia7d516c2cf3a51f5a657a3aad76188041dcc6620

Both Khronos and we say to use < > in our docs

Change-Id: Ibf1d2b83036582a605aa6b9043210c89a4eb0e1e

These AudioTrack operations are relatively expensive Binder calls (and
will soon be even more expensive to fix a deadlock at AudioTrack::start),
and they were being called excessively.  Now AudioTrack start, stop,
and pause are only called when there is an actual play state change.

Details:
 - distinguish ATTR_TRANSPORT vs. ATTR_PLAY_STATE for audio players
 - android_audioPlayer_setPlayState is only called when the play state changes
 - handler_AudioPlayer_transport is only called for transport changes other than play state
 - android_audioPlayer_setPlayState is always called with mutex locked
 - since media player currently only handles play state, handler_MediaPlayer_transport
   is still called for both ATTR_TRANSPORT and ATTR_PLAY_STATE
 - since the handlers for ATTR_BQ_ENQUEUE and ATTR_ABQ_ENQUEUE are only called if in
   state PLAYING, changed an "if" to an "assert"

Change-Id: Iee2968fd98d215885b7105053bb1604f962ea337

Attribute handler functions are more modular and
get rid of those endless if statements.

Change-Id: I3221aeb3d26d2dcc5dc9a3f6d37d088f5ce511a1