Remove enabled flag compute_window_changes_on_a11y_v2

It has been enabled 6 month.

Note that AccessibilityWindowManagerTest is actually removed, and
AccessibilityWindowManagerWithAccessibilityWindowTest is renamed to
AccessibilityWindowManagerTest.

Bug: 322444245
Test: build, atest AccessibilityWindowManagerTest
Flag: EXEMPT removing com.android.server.accessibility.compute_window_changes_on_a11y_v2
Change-Id: Ifd8328fd21f5547726313c65c60d2a4b0d6095af