Settings: disable hardware unlock buttons while secure

Disable the unlock home/menu/camera options in settings
while a secure lockscreen is active to signal they do
not work to the user.

Change-Id: I193d253d40c7ceef80c6659b08d02be7159f476d
Signed-off-by: Roman Birg <roman@cyngn.com>
Merge "Settings: disable hardware unlock buttons while secure" into cm-11.0