Forwards onConfigurationChanged for PiP on reparenting

This fixes a regression caused by ag/5656236

PiP saves its last position on screen within
AppWindowToken.onConfigurationChanged callback, which is triggered by
Task.reparent before the task leaves pinned stack.

With ag/5656236, Task.reparent no longer triggers
AppWindowToken.onConfigurationChanged and instead, the latter will be
called when the task is added back to its original stack while the
pinned stack no longer exists.

Bug: 142504739
Test: atest PinnedStackTests#testEnterPictureInPictureSavePosition
Change-Id: I247a9da3113077031b442740445d836b0c176986