Adds a method to call into WMShell to set the listener and
indicate when a bubble is shown or when the expanded state
changes.
BubbleBarController sets or clears out the listener.
BubbleBarViewController calls SystemUiProxy when bubbles are shown
or when the expanded state changes.
Bug: 253318833
Test: manual, with other CLs, see go/bubble-bar-tests
Flag: WM_BUBBLE_BAR
Change-Id: I2364b9bbdea237fc268b0999d9c896585c194a86
Mady Mellor
50e8d0ff60