Continuous zooming/panning while Magnification shortcuts are pressed

As long as the repeat keys feature is enabled, holding down the
magnification keyboard zooming/panning shortcuts will cause
continuous panning / zooming (until the limits are reached).

As a follow-up, MagnificationController can be informed when
repeat keys setting changes, and can handle diagonal
zoom/pan.

Test: atest MagnificationControllerTest, manual
Bug: b/388847283
Flag: com.android.server.accessibility.enable_magnification_keyboard_control

Change-Id: I21bdc25b6c08e51cebfa168bd88abdfda530960d