Show system-fixed permissions but do not allow them to be disabled.

Instead of not showing system-fixed permissions as we did before, we
now show them.  The controls are disabled so users cannot disabled
them, and we show a short message explaining this.

Bug: 119622068
Test: See disabled toggles for system-fixed permissions in
AppPermissionFragment, AppPermissionsFragment, and
PermissionAppsFragment.

Change-Id: I6805ba7bf4736c2c7561ffdbd30178d1a4243b45