AudioEffect acquires session

Currently, users of audio sessions, AudioTrack and AudioRecord,
 are acquiring and releasing audio sessions according to their
 life-cycle. AudioEffect instances were not counting as users
 of an audio session. This caused an effect used on a session
 to be purged by AudioFlinger::purgeStaleEffects_l() whenever
 the last user of that session went away.
This CL makes AudioEffect acquire and release a session when
 created and destroyed.

Bug 15432115

Change-Id: I922532150009988d43872f9b5928044a830ae0b3