The magnification mode is full-screen when the migrating deivce turns on
magnification settings.
We show a notification prompt the user that new window magnifier to
guide the user to take a look.
Bug: 168635084
Test: 1. adb shell settings put secure \
accessibility_show_window_magnification_prompt 1
2. use full-screen magnification to see if it works well.
atest WindowMagnificationPromptControllerTest
atest FullScreenMagnificationGestureHandlerTest
Change-Id: I40dfc49307d2dad380f1e662960d35f5cc5e7524
3badad790d