Notifies window magnifcation changes for AccessibilityService (1/n)

See Design doc:  go/b200769372
To make the behavior consistent on the new platform,
The legcay callback,
  onMagnificationChanged(MagnificationController controller,
  Region region, float scale, float centerX, float centerY)
keep notifying only full-screen magnification change as
before.

To support listening to the magnification changes of all
magnification modes, the service should overide
The new callback proposed in T,
  onMagnificationChanged(MagnificationController controller,
  Region region, MagnificationConfig config).

TODO: Notify magnifcation change when window magnifier turns off

Bug: 203013925
Test: atest AccessibilityMagnificationTest,
    atest MagnificationControllerTest,
    atest WindowMagnificationManagerTest,
    atest WindowMagnificationControllerTest,
    atest FullScreenMagnificationControllerTest,

Change-Id: Ia73195a71f55bb26ef3f2eafce0dc73d8a65583b