Fix activity launch incorrectly blocked after activity finishes

  A race condition can cause the activity security model to incorrectly
  block a legitimate activity launch. This occurs when a top activity
  finishes and another is launched immediately after. When the first
  activity's task is removed, it can leave a stale reference in
  TaskDisplayArea.mPreferredTopFocusableRootTask.

  Subsequently, when getFocusedRootTask() is called to determine the
  current focus state, it returns this stale task. The
  BackgroundActivityStartController misinterprets this, believing a new
  activity is trying to launch over a valid focused task, and blocks it.

  The necessary cleanup logic to clear mPreferredTopFocusableRootTask
  was previously guarded by the polish_close_wallpaper_includes_open_change
  flag. This change removes the flag check, making this cleanup
  unconditional.

  This ensures the focus system always has an accurate view of the task
  stack and prevents the security model from making decisions based on
  stale data.

Fixes: 429317375
Test: atest CtsWindowManagerBackgroundActivityTestCases:android.server.wm.ActivitySecurityModelTest#testTopFinishesThenLaunchesActivity_launchAllowed
Flag: EXEMPT BUGFIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:836a80bc1cbc72b22cb79d2594ed088c6b58581c
Merged-In: I77dbe773723cb6a3e5bc568a57040c469c1263c2
Change-Id: I77dbe773723cb6a3e5bc568a57040c469c1263c2