Camera: Improve metadata access JNI performance

- Don't use reverse-JNI to access metadata ptr, pass it as an arg instead
- Use @FastNative since the calls are short and bounded in time

Performance improvement:
- On a 10-second trace of camera app running on sargo, percentage of
  time used in CameraMetadataNative.get went from 4.36% to 3.77%, a 15%
  reduction in time.

Test: atest CtsCameraTestCases
Bug: 150214459
Change-Id: I28d9428beaa7eada6292e24fe6ca1dbd9c2ff153