camera2: Update Face class.

- Move Face to be outer class. Enables Parcelable-izing it later.
- Add static public constants for field values.
- Add @hide constructors.
- Rename methods returning Point to have Position suffix.
- Add new key android.statistics.faces (CaptureResult#STATISTICS_FACES)

Bug: 10360518
Bug: 10549293
Change-Id: I067f06f0426114b2c3a3266ca7e00e6cb1d89046