Fix camera facing by querying characteristics

Fix camera facing by querying characteristics

Query facing from characteristics instead of hard coded.
Some code like camera_id_icons still assume the back camera id is
0. But these codes are only used when switching cameras. If there
is only one camera (either front or back), camera app works fine.
If there are two cameras, back camera id should be 0.

Also changing CHECK_BACK_CAMERA_ONLY to false because some devices
only have a front camera.

BUG=27516910
TEST=manually run camera app on device

Change-Id: I9f6914e2eb7609a6cb7eb840e3e27e440a7c81df
(cherry picked from commit daa06b80d28adfccfb785b8fa9b73898af50579b)