stagefright: fix potential codec leaks in async stop/reset

Currently if stop is followed by a reset from another thread,
we unblock the stop first (to avoid ANR) without waiting for
kWhatShutdownCompleted, and proceed with the reset request.
But this could cause release to return too early when the
kWhatShutdownCompleted for the stop is received. When this
happens, the reset() will unregister ACodec's handlers before
the real shutdown with deallocation is processed, causing
a codec node instance to be leaked.

To solve this problem, separate kWhatShutdownCompleted into
two messages kWhatStopCompleted and kWhatReleaseCompleted.
Release should only return upon kWhatReleaseCompleted.

Also initializing the fence fd in the metadata buffer (even
though it's not used) to avoid warning in freeBuffer.

bug: 32750454
bug: 32971559

Change-Id: If218c853955729a2a2f2e8356908115a79c442e3