camera2: Add key enumeration functionality to CameraMetadata

* Add a way to enumerate all currently set keys in CameraMetadata
* Add a way to enumerate all available keys for CaptureRequest
* Add a way to enumerate all available keys for CaptureResult
* No way to enumerate all keys for CameraProperties, since it would be
  identical to all the currently set keys.

Bug: 10360518
Change-Id: I3a90f8cc385db14a675e4ff876ae93d906ff06bf