fw: reintroduce PRIVATE_FLAG_PREVENT_POWER_KEY

Switching the PREVENT_SYSTEM_KEYS forces the application to handle
every system key.  Some apps may not want to handle every single key,
so allow them to just handle the power key.

CYNGNOS-2958
Change-Id: I43fd65e67b909e6d52548f110ff9997b829b3e15
(cherry picked from commit 44e91e0e)