Camera: Powerkey shutter (2/2)

Ported from cm-10.1

Including cm-10.1 camera commit:
aa4ae80a41fbab763891c5ef6d67a9e5b4bb981b Don't mess around with pref local ID for power shutter initialization.

Includes http://review.cyanogenmod.org/56986
PS2: Make Camera2 a privileged app
PS5: Sign with platform certificate instead of making it a privileged app.
PS6: Add write permission to PREVENT_POWER_KEY
PS7: Move PREVENT_POWER_KEY permission to CameraActivity
PS8: Revert to PS5

Change-Id: I20523fd8259fc22f07c5a5cf016e339fbaf46969
Signed-off-by: Michael Bestas <mikeioannina@gmail.com>