Enable App Ops under security setting.

Squash of two commits:

Add support for MODE_ASK mode and enable App Ops under security settings.

This is the 2nd commit message:

AppOps: Fix previous conflict resolution bug

Fix merge conflict resolution issue caused by
19ae20d2149e5898927eba460d1656a7509c53ab.
"toggle_install_applications" should be
SwitchPreference.

Change-Id: I36b43af8a75f0ba6cebc24c441474a0e9a9dc963