Unhide getOrCreateThreadId

Multiple components need to lookup thread IDs and use getOrCreateThreadId.
Examples: Contacts, Glass, Hangouts, Messenger, voice search and bluetooth.

In several cases getOrCreateThreadId is used by unbundled apps.
These apps go far out of their way to use getOrCreateThreadId, because
it fills a hole in functionality. Note how glass, voice search and
Hangouts use reflection to access getOrCreateThreadId.

We should unhide this method.

Bug: 18777272
Change-Id: Id3bb1e3f73c4826154096057a778ac97ee86abc6