Eliminate potential deadlock in AccessibilityCache

The cache calls out with its lock to the app main thread when refreshing
nodes. This opens the door for a deadlock, and causes a slowdown for
non-main threads.

Bug: 180957109
Test: atest CtsAccessibilityServiceTestCases  CtsAccessibilityTestCases
CtsUiAutomationTestCases
FrameworksServicesTests:com.android.server.accessibility
FrameworksCoreTests:com.android.internal.accessibility
FrameworksCoreTests:android.view.accessibility

Change-Id: I27e4e64f778ae1c2a348e16cf8739f7a5596e0fc