Camera2: return CaptureResult.STATISTICS_FACES

Changes included in this CL:

- remove the WAR for face detect mode null case.
- make the sanity check assert to warnings.
- make number of faces to be the minimal length of face metadata array length to
avoid out of bound access, since we relax the sanity check.

Bug: 11206459
Change-Id: Ifd9f7f79617039c1b3ad631f202b48076d775e09