GPS: remove GpsEventThread from GpsLocationProvider

Rather than polling for events from the native code in an event thread,
we now require the GPS HAL libraries to call our callbacks from a thread
that is registered with the JVM to call directly into Java.
This eliminates a thread from our code and removes one step in the chain
of message passing from the GPS to the Location Manager client.

Change-Id: I2745a157690310ba9a699a8369f54a7366c6b1ba
Signed-off-by: Mike Lockwood <lockwood@android.com>