[PiP on Desktop] Fix PiP issues with multiple_desktops_backend flag

This CL fixes a few issues with multi-activity PiP when the
multiple_desktops_backend flag is enabled.
- Instead of wct.reorder, the parent task for the multi-activity PiP
  needs to be minimized instead. In addition, when expanding PiP, the
parent/original task then needs to be unminimized.
- With the flag enabled, the child task that is newly created for the
  PiP window is reparented to the DeskRoot in Task#validateRootTask.
When InputMonitor updates the input consumer for the PiP window, the
root task's bounds are used, which ends up being fullscreen in this
case, causing PiP to consume input on the entire screen. This CL
removes the initial Builder#setParent call and adds a
TaskDisplayArea#addChild call after build() is called to ensure the new
pinned task is added to the TDA.

Bug: 419621266
Bug: 419908475
Bug: 421205793
Test: atest DesktopTasksControllerTest
DesktopPipTransitionControllerTest
Test: atest WMShellUnitTests:com.android.wm.shell.pip2
Test: atest WMShellUnitTests:com.android.wm.shell.pip
Test: atest WMShellUnitTests:com.android.wm.shell.common.pip
Test: Manual - launch ApiDemos in Desktop session and verify entering
PiP and expanding is WAI, and PiP window is only consuming input within
its bounds. Also verify that expanding PiP is WAI.
Test: Manual - launch ApiDemos in CD with empty desk. Verify PiP enters
on minimize and expanding PiP is WAI.
Flag: com.android.window.flags.enable_desktop_windowing_pip

Change-Id: I36971db0f86dec8312ba42d6b2ab60ba0c6e0e42