[A11y Focus API] Add null AttachInfo check in View#onFocusChanged

* Add null check to fix the flagged logic that is preventing flag
advancement in ag/33178285.

Flag: android.view.accessibility.request_rectangle_with_source
Bug: 391877896
Test: Manually confirmed fix for AccessibilityWindowReportingTest#testGetAnchorForDropDownForAutoCompleteTextView_returnsTextViewNode
Change-Id: I2253bc657e6cf0e766d9dea6b79715a2885e335c