camera2: Fix available keys to filter to only supported keys

The following functions are affected:
* CameraCharacteristics#getKeys
* CameraCharacteristics#getAvailableResultKeys
* CameraCharacteristics#getAvailableRequestKeys

The above keys will now only return keys whose tags are set in
android.request.available{Characteristics,Result,Request}Keys

This change makes it possible to detect if a key is guaranteed to be
available at a granular basis (without querying hardwareLevel or
capabilities).

Change-Id: Iaa34e8c24100339c215604ff49035527fae467cd