Camera: Validate face count in received metadata

Ensure the count can't cause an overflow in bytes to be read.

Test: atest CtsCameraTestCases; also add bad face count data from camera
  service and manually verify the error logs appear when running
  android.hardware.cts.CameraTest#testFaceDetection.
Bug: 150156131
Change-Id: Ic78ec0ccf67ef8665f80f69aabbb1ae71dd609cd