Wait until GpsLocationProvider is fully initialized before returning from the constructor.

Otherwise we are left with a race condition that might result in the provider
being called before it is ready.

BUG: 2539392
Change-Id: I05f4a2501c43d5fa95c0e68f5a963015cf9c6e8f
Signed-off-by: Mike Lockwood <lockwood@android.com>