Add a test for stashed bubble bar

This change introduces a new test protocol event for unstashing the bubble bar.

It's not possible to do this via a UI event because the stashed handle is only visible when an app is in the foreground, but in that state none of the elements in TaskbarDragLayer are visible to accessbility.

Flag: LEGACY persist.wm.debug.bubble_bar DISABLED
Bug: 273994695
Test: atest PlatformScenarioTests:android.platform.test.scenario.sysui.bubble.BubbleBarTest
Change-Id: I4581bf95b07691429966ecbd573232047f820b69