camera2: Refactor CameraMetadata.Key out into 3 key classes

Before:
* CameraMetadata.Key<T>

After:
* CameraCharacteristics.Key<T>
* CaptureResult.Key<T>
* CaptureRequest.Key<T>

CameraMetadata#get has been removed (each metadata subclass has
its own #get now) due to java generic limitations (in particular
a type bound <T1<T2> extends Key<T2>> is an illegal bound).

CameraMetadataNative gets a new #dumpToLog function to dump the native
metadata to logcat.

Bug: 15091017
Change-Id: Ic56c54c0d184e209e20de374dc8a6d79527c209f