Implement showDesktopApps

Copy logic over from proto1 to bring desktop apps to front.
There is one key difference from proto1 logic. In proto2, before desktop
tasks are shown, we ensure that the home task is brought to front. And then
place desktop tasks on top. This is due to proto2 having fullscreen and
freeform tasks running at the same time.

Bug: 260645044
Test: atest DesktopTasksControllerTest
Change-Id: Ic5de4d431b96893ef5d685a4dce28e08f17f6f97