SystemUI: write keyguard enabled setting to CMSettings

Instead of saving this to a shared preference in SystemUI, let's
leverage the CM SDK Settings Provider so we can query this state and let
the user know why the keyguard is disabled.

Also move the logic into KeyguardViewMediator to monitor the state
change and apply it locally.

Ref: CYNGNOS-1513

Change-Id: I08a984797921178082e1e7bda881b7b8a7965ddf
Signed-off-by: Roman Birg <roman@cyngn.com>