Cleanup AccessibilityWindowManager#onWindowsForAccessibilityChanged

Due to a flag compute_window_changes_on_a11y_v2,
AccessibilityWindowManager.DisplayWindowsObserver has two entry points,
one is #onWindowsForAccessibilityChanged (for flag disabled) and another
is #onAccessibilityWindowsChanged (for flag enabled).
Now that the flag is removed [1], we can merge them to simplify the
code.

[1] Ifd8328fd21f5547726313c65c60d2a4b0d6095af

Bug: 322444245
Test: atest AccessibilityEventAndCacheTest
Flag: EXEMPT cleanup after removintg com.android.server.accessibility.fix_merged_content_change_event_v2
Change-Id: Ia72589b03c6af12abb8cc990325303f3afed23ea