Add a public API defaultFocusHighlightEnabled.

This commit adds a public API to turn on/off default
focus highlights. When a focused View is detected to
have no state_focused defined, we can use this API to
decide wether we should show a default highlight for it.

Test: cts-tradefed run singleCommand cts --skip-device-info
--skip-preconditions --abi armeabi-v7a -m CtsViewTestCases
-t android.view.cts.View_DefaultFocusHighlightTest
Bug: 35096940

Change-Id: I76b45d6bf5761641a0ed7f4d0b04cb325ed72b52