Add AccessibilityDisplayProxy and register/unregister methods

An A11yDisplayProxy will be used by Exo to listen to UI changes and
perform actions on the UI.

For a Display that is shown on a ChromeBook/client device,
a privileged app like Exo, or an app with SystemAPI privileges and
the MANAGE_ACCESSIBILITY permission, should call
A11yManager.registerA11yProxy and A11yManager.unRegisterA11yProxy to
enable/disable a11y.

The app will only have visibility in the specified proxy display.

Currently only few A11yProxy methods are exposed. When the proxy is
plugged into the system via ProxyAccessibilityServiceConnection
other methods will be exposed. (See link in comments)

Bug: 241429275
Test: atest A11yProxyTest(cts), atest A11yManagerTest(unit)
Change-Id: I019732e50e6beba0af7f5898a07b44d25e03416b