Hide UI without flashing
This branch fixes the "flash" that happens when the UI is hidden due to low memory, by making HideUiActivity a subclass of Activity so it can use the NoDisplay style. The same change is applied to ExitActivity. PanicResponderActivity is also meant to be invisible, but needs to remain a subclass of AppCompatActivity, so it's changed to use a translucent theme.
Closes #1469 (closed)