Camera1 shim: If there's no flash, the keys need to be null.

Previously, we set FLASH_MODE_OFF for FLASH_MODE when a flash unit
isn't available.  However, per the API documentation, the key has to
be null instead.

- Make sure that the flash mode and supported flash mode keys are null
  if there's no flash unit on start
- Don't set flash mode in later setParameters calls if there is no
  flash unit
- Map NULL value for flash mode key to FLASH_MODE_OFF for internal
  consistency.

Bug: 17660716
Change-Id: I3033682f0b882b8c2004114e2afef31662caebda