Cleanup some of the thread merging.

Adds an optimization for checking whether a looper is stuck,
with a new Looper method to see if its thread is currently
idle.  This will allow us to put a large number of loopers
in the monitor efficiently, since we generally won't have to
do a context switch on each of them (since most looper threads
spend most of their time idle waiting for work).

Also change things so the system process's main thread
is actually running on the main thread.  Because Jeff
asked for this, and who am I to argue? :)

Change-Id: I12999e6f9c4b056c22dd652cb78c2453c391061f