[PiP on Desktop] Minimize multi-activity PiP for non-Shell transitions

For content PiP, the PiP transition is not started through
DesktopTasksController#minimizeTask, so the multi-activity PiP parent is
not being minimized as it should. This CL adds the minimization through
DesktopPipTransitionController#handlePipTransition so content PiP
multi-activity case is handled.

Bug: 422711549
Test: Manual - visit DailyMotion on Chrome, play a video and trigger the
native PiP function on the video player. Verify PiP enters and the
Chrome task is not seen.
Test: atest DesktopPipTransitionControllerTest
DesktopTasksControllerTest
Flag: com.android.window.flags.enable_desktop_windowing_pip

Change-Id: If676dbfb1e4ba855d500abb232c2191823825ee0