a11y: Magnify the Magnification dialog

This CL will enable Magnification feature when the dialog shows up; if
the user clicks the positive button, we will do nothing to keep
Magnification on; if the user clicks the negative button or dismiss the
button, we will remove the shortcut from user state.

Bug: b/425722546
Flag:com.android.hardware.input.enable_magnify_magnification_key_gesture_dialog
Test: manual & atest AccessibilityManagerService,
KeyGestureDialogStartableTest

Change-Id: I1d598532a3af44d0688f12969a793f80d12802d8