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
diff --git a/include/media/AudioEffect.h b/include/media/AudioEffect.h
index f98002d..4932d40 100644
--- a/include/media/AudioEffect.h
+++ b/include/media/AudioEffect.h
@@ -449,6 +449,7 @@
     sp<EffectClient>        mIEffectClient;     // IEffectClient implementation
     sp<IMemory>             mCblkMemory;        // shared memory for deferred parameter setting
     effect_param_cblk_t*    mCblk;              // control block for deferred parameter setting
+    pid_t                   mClientPid;
 };