Add a secure setting for double twist gesture for camera flip.

Add Secure.CAMERA_DOUBLE_TWIST_TO_FLIP_ENABLED to control whether the
double twist gesture should flip the camera between front and back mode
is enabled or not. Also move SYSTEM_NAVIGATION_KEYS_ENABLED from
Global to Secure.

Bug: 29869445
Change-Id: I614289e6de7b1ee4221de7047a5e0dd784607bed