Small cleanup AccessibilityWindowManager#updateWindowsLocked

After a flag compute_window_changes_on_a11y_v2 is removed [1], a
method #populateReportedWindowLocked returns non-null. Thus we can
simplify some window computation in A11yWindowManager.

[1] Ifd8328fd21f5547726313c65c60d2a4b0d6095af

Bug: 322444245
Test: atest AccessibilityWindowManagerTest
Flag: EXEMPT cleanup after removing com.android.server.accessibility.compute_window_changes_on_a11y_v2
Change-Id: Idbb99e276b06cd742d0e3bef96171852d41cbdd7