Refactoring of the screen magnification feature.

1. This patch takes care of the case where a magnified window is covering an unmagnigied
   one. One example is a dialog that covers the IME window.

bug:7634430

2. Ensuring that the UI automator tool can connect and correctly dump the screen.

bug:7694696

3. Removed the partial implementation for multi display magnification. It adds
   unnecessary complexity since it cannot be implemented without support for
   input from multiple screens. We will revisit when necessary.

4. Moved the magnified border window as a surface in the window manager.

5. Moved the mediator APIs on the window manager and the policy methods on the
   WindowManagerPolicy.

6. Implemented batch event processing for the accessibility input filter.

Change-Id: I4ebf68b94fb07201e124794f69611ece388ec116