Allow updating the flyout message

Add functions to allow updating the contents of the flyout while it
is either expanding or fully expanded.

Flag: com.android.wm.shell.enable_bubble_bar
Bug: 277815200
Test: atest BubbleBarFlyoutControllerTest
Change-Id: Iea3fc7df792e02605df6b44c1da39e267f6d9d43
3 files changed