Expose A11yDisplayProxy#setFocusAppearance

A proxy can prevent an app's accessibility focus from being drawn, to
avoid conflict with its own focus outline (For example, an Exo screen
reader should have its own focus).

Bug: 254545943
Test: manual test with non-default display, atest AccessibilityDisplayProxyTest
Change-Id: Ib794de54490aa53f5d478fdba00dbd9578982a47