Preserve bubble bar state.

Added logic to save and then restore existing bubbles in the
TaskbarSharedState object.

Bug: 375482371
Test: Manual. Switch between pinned and unpinned taskbar. Video:
http://recall/-/gx8ASgewUeUS3QYohfrd1J/d8hVHxksAokx85J8P7etoC
Flag: com.android.wm.shell.enable_bubble_bar
Change-Id: I34f580f1c652859f427385edc0ccadd447e1400f