Make vendor keys visible in Camera2 java APIs.

- Add vendor keys to getKeys() calls for CameraCharacteristics,
  CaptureRequest, and CaptureResult.
- Vendors can specify whether custom keys show up by listing
  visible keys in the REQUEST_AVAILABLE_RESULT_KEYS field.
- Vendor key types are always treated as a primitive (or Rational)
  array type corresponding to one of the valid types for
  a camera metadata entry.

Bug: 22067625
Change-Id: I6e7dd3db7a8bf533c2ec15ff69ca38824134e971