update secure keyguard check

We use this method to determine wither to allow profiles and the lock
screen tile whether to allow messing with the lockscreen.

Don't check whether the keystore is empty. Check whether we allow trust
agents by DPM. And also make keyguard use this method to respect it.

Ref: CYNGNOS-1930

Change-Id: I1a2b26419d43bd2bbc1d4a5027f1ed78d2744466
Signed-off-by: Roman Birg <roman@cyngn.com>