Add method to request window magnification connection

This method is used by Accessibility framwork to request
window magnification connection. When it is called, the connection will
be set by AccessibilityManager#setWindowMagnificationConnection.

Bug: 136250281
Test: IWindowMagnificationConnectionTest, WindowMagnificationTest
Change-Id: Ib4b3c6d7cf90a6c2c1d26029f990285466ff6419
8 files changed