Ensure ViewRootImpl.setAccessibilityFocus() doesn't crash when reentrant.

Internal state must be cleared before calling any methods on the focus
host, since the method may be called again from the host and attempt to
recycle the same AccessibilityNodeInfo twice.

BUG: 8856860
Change-Id: I0410989fd6f3ce3ce29de8edebdfbf3847188843