Add work source uid collection to LooperStats

Add ThreadLocalWorkSourceUid and Message.workSourceUid
which will store the UID which caused, directly or
indirectly, the Message to be enqueued.

This was needed because using Binder.getCallingUid() does
not give us enough data since quite a few Binder services
call Binder.clearCallingIdentity() when processing the
call.

Test: UT and manual
Change-Id: I35af3a9ca5193477070990b41ff5d9c52f420069