Torch: rewrite WhiteScreen functionality

  1. Handle it inside TorchActivity instead of creating
     a new activity unnecessarily

  2. Exit white screen by pressing Vol+ or Vol- key

  3. Make WhiteScreen immersive

  4. Allow coming into WhiteScreen for devices that
     have hardware torches too (using an extra
     Boolean in the intent called "whitescreen")

Change-Id: I5a30029dde0035ec98c8258ad382cda74deb7378
Signed-off-by: Arnav Gupta <championswimmer@aokp.co>