Add new native Looper API.

This allows us to avoid exposing the file descriptor of
the event queue; instead, you attach an event queue to
a looper.  This will also should allow native apps to be
written without the need for a separate thread, by attaching
the event queue to the main thread's looper and scheduling
their own messages there.

Change-Id: I38489282635895ae2cbfacb88599c1b1cad9b239