fix(fullscreen magnification): Create connecting methods between SystemServer...

fix(fullscreen magnification): Create connecting methods between SystemServer and SystemUI for fullscreen mode

1. Creating the methods onFullscreenMagnificationActivationChanged to notify the fullscreen magnification activation change.
2. Allow establishing the magnification connection in fullscreen only mode.

Bug: 291891390
Test: adb shell device_config put accessibility com.android.window.flags.magnification_always_draw_fullscreen_border true
      atest MagnificationConnectionManagerTest
      atest MagnificationControllerTest
      atest MagnificationConnectionWrapperTest
      atest AccessibilityManagerServiceTest
      atest frameworks/base/services/tests/servicestests/src/com/android/server/accessibility/magnification
Flag: ACONFIG com.android.window.flags.magnification_always_draw_fullscreen_border DEVELOPMENT
Change-Id: I0bae5a074af6c0546bdb204c34656182d686a0df