Added an interface between Sysui and system_server

This interface is used to interact between system UI and system
server for winodow magnification. When magnification mode is window
mode, system UI will set the connection to AccessibilityManagerService
to manipulate window magnification.

Bug: 146400227
Test: WindowMagnificationConnectionWrapperTest
Test: AccessibilityManagerTest
Test: WindowMagnificationManagerTest
Change-Id: I64b22eac3a7be2e1dbced0573988fe75ace921f6