Replace inappropriate SupervisionManager property accessor usage with regular method calls.

"Property accessors should not have any kind of side effect, and should not be able to throw on access" - but all of these replaced SupervisionManager methods can throw and some have side effects.

Flag: android.app.supervision.flags.enable_supervision_settings_screen
Test: atest
Change-Id: Ie4e1c1bb1364e582a1d7463c34d94cc6a85820c4