Make some small fixes to sensor privacy ui

Allow dialog to show on lock screen.
Force the user to make a decision so they can't accidentally leave the
dialog and miss the opportunity.
Protect the changing of state using quicksettings by the keyguard.
Protect disabling sensor privacy with the dialog by the keybguard.

Test: Manual
Bug: 162549680
Fixes: 178759375
Change-Id: I481eea900b2a9a25c700b847c5090ea64855d55d