Query the cache for AccessibilityInteractionClient's findFocus

TalkBack is making a lot of findFocus calls which is impacting
performance. We track focus in the cache so reuse it here.

AccessibilityManagerService does not need a cache, so
only initialize a cache when needed

Bug: 187731774
Test: atest CtsAccessibilityServiceTestCases  CtsAccessibilityTestCases CtsUiAutomationTestCases FrameworksServicesTests:com.android.server.accessibility FrameworksCoreTests:com.android.internal.accessibility FrameworksCoreTests:android.view.accessibility
 atest AccessibilityCacheTest, manual
Change-Id: Ie6f693345d31a3f82ffa479428f76a09fa972a83