API for audio session ID generation

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