Expose the value used by the framework to generate an audio
session ID when a value isn't known.
Rename allocateAudioSessionId() to generateAudioSessionId()
as this operation doesn't "allocate" anything, and there is
no allocated resource to free after this operation.
Bug 16401631
Change-Id: I7a7bc05b39ea0b024ff225254eb755a9c85a2ad9