Expose window type MAGNIFCATION_OVERLAY to AccessibilityService

This window type is for distinguishing the windows used for
magnification. Accessibility apps could move the window magnifier
to follow accessibility focus, but they need to exclude the case that
the focus is on the window magnfier with this information

Bug: 188136191
Test: manual test , will add cts when the public api is ready
Change-Id: Ic2598d25ead5b1d74ebb2c0ea181bfe139675750