Fix zombie threads in the Layoutlib.
Some widgets can create (indirectly) HandlerThread which are started
but never stopped.
This patch use the delegate methods to override the thread run method
to record which threads are started to be able to call quit() on them
after a render.
Note that we should really fix layoutlib_create to be able to call
the default implementation from a delegate to remove code duplication.
Change-Id: Iaf5432623e9704bb3114a8e0583c8553282c205e
156a211b21