jni: GpsLocationProvider: Check for pending callbacks before waiting

This change fixes a corner case where a callback may not get handled until
a second callback arrives.  This can happen because there is a significant
section of the wait_for_event function where the mutex is not locked, and
the sPendingCallbacks member could be updated.  We now check to see if
there is a pending callback to handle before we wait for another callback.

Change-Id: I20cfae1e780944bb74133940dda032efc4c55540
Signed-off-by: Fred Fettinger <fred.fettinger@motorola.com>
Signed-off-by: Jared Suttles <jared.suttles@motorola.com>
Signed-off-by: Mike Lockwood <lockwood@android.com>