Handle swipe down on collapsed bubble bar

Allow swiping down on collapsed bubble bar to stash it.

Bug: 325673340
Test: atest NexusLauncherTests:com.android.launcher3.taskbar.bubbles.BubbleBarSwipeControllerTest
Test: swipe up to show bubble bar and swipe down to stash it
Test: tap on stashed handle
Test: enable 3 button nav and tap on bubble bar
Flag: com.android.wm.shell.enable_bubble_bar
Change-Id: I0367d36bdc1aef34dc71843052b691a95e293375