Attempt to make waitForIdleHandler reliable.

The current implementation of IdleableHandlerThread is based on
the assumption that MessageQueue#isIdle will return true iff the
message loop has finished processing its messages.

Unfortunately, this is incorrect: isIdle returns true iff are no
more messages waiting in the queue; thus, it will also return
true while it is processing the last message before going idle.

Instead of using idle handlers, take the simpler approach of
posting a runnable to the message loop and waiting for it to
be processed.

Test: see other CLs in this patch series
Bug: 31479480
Change-Id: Iae75781f067b762c8653a488a5e4d5ee0c789e01