Ensure onStop before onPictureInPictureModeChanged

We have a CTS test case
PinnedStackTests#testStopBeforeMultiWindowCallbacksOnDismiss that
ensures onStop is always sent before onPictureInPictureModeChanged. The
test itself does not simulate what user typically does and it's broken
with the shell transition.

Apps like YouTube may rely on such order so they can choose if they
should, for instance, stop the playback when the PiP is removed.

Fixing this by forcefully send onStop before setWindowingMode (which in
turn results a configuration change).

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/eEEfu9mca0dzLyzC8jBcxb
Bug: 290413333
Test: manually, see also the video
Test: atest PinnedStackTests
Change-Id: I0f94d4c26f7b3ccc1df6be97a0238f995438dffd