Creates BubbleBarView & BubbleBarViewController & friends

BubbleBarView shows BubbleViews in a bar similar to transient
taskbar. BubbleBarView can be collapsed (bubbles in a stack) or
expanded (bubbles all visible). When expanded, WMShell will be
notified to show the appropriate expanded bubble view (not part of
this CL).

Also creates BubbleControllers which contains BubbleBarViewController
and will eventually contain other controllers related to stashing
for the bubble bar.

Bug: 253318833
Test: manual, with other CLs, see go/bubble-bar-tests
Flag: WM_BUBBLE_BAR
Change-Id: I990ab3da6614db90ffff8c40281dc7f16b3957f6