audioflinger: Pass pid of process creating track or opening record

AudioFlinger allocates a Client heap for each unique pid.
If two applications use mediaplayer APIs, the same Client heap
is reused as the pid used is not the application pid but that
of mediaserver. With this change, the pid of the application
pid is used to decide the heap to be used.

Bug: 23525542
Bug: 28772898
Change-Id: I31a695b0b321eff6e2aca80c3bc4aeb3e1cd9ac7