Simplify ThreadBase::exit() aka requestExitAndWait

We can remove mExiting and use Thread::exitPending() instead.

The local sp<> on "this" in exit() is not needed, since the caller must
also hold an sp<> in order to be calling us. (Unless it was using a raw
pointer, but that would be dangerous for other reasons.)

Add comment explaining the mLock in exit().

Change-Id: I319e5107533a1a7cdbd13c292685f3e2be60f6c4