Avoid reparenting the existing task if the launch is blocked

The activity launch was blocked by BAL, however, the existing task
was reparented to the launch-root-task. In the issue case, the task
was reparented from minized-desk to desk root task.

Bug: 434047695
Test: verified via the sample app
Test: ActivityStarterTests
Flag: com.android.window.flags.fix_bal_reparent_existing_task
Change-Id: Ieaa225dc63874c58a5d6bed7b0c6ecad4acdb4ec