camera2-portability: Support photo capture using camera2 API

This implements JPEG capture, including an autoexposure precapture sequence.
There are many changes to AndroidCamera2Capabilities and AndroidCamera2Settings
to support the representation of modes (e.g. flash modes) whose flags do not
map trivially between the API implementations. Part of this work is the
conversion of AndroidCamera2AgentImpl to use and store a
Camera2RequestSettingsSet instead of a bare API 2 CaptureRequest.Builder.

Change-Id: I03f9f98c954a7b0c140ac8d80161878c92ef65d2