Only clear preferred UID at appropriate times.

The preferred UID is set when we call
JobServiceContext.cancel(...PREEMPT), so clearing the preferred UID
before that call is pointless. Also, make sure not to clear the
preferred for contexts that are only going to be used to transfer jobs
to the preferred UID.

Bug: 141645789
Bug: 204924801
Test: Android builds
Change-Id: I18db34a389919432b604d68f90a6901d9e6f9cba