Allow fullscreen magnification keep zoom level on context change

This change allows devices to customize the fullscreen magnifier
behavior so that when context change happens, for example opening a new
activity or closing, magnification won't be zoomed off.

Flag: EXEMPT only enabled on devices that has the config
Bug: 352407978
Test: FullScreenMagnificationControllerTest
Test: Enable config and play around settings activity and PlayStore
Change-Id: I0f801f5162ce8bf06e86424bc0885271d4e1c397