Handle WM Shell crash in MultitaskingController

If the multitasking controller delegate goes away because of WM Shell
process death, report to the client that all Bubbles it requested
were removed. Also allow registering a new delegate when SysUI comes
back.

Bug: 407149510
Flag: com.android.window.flags.enable_experimental_bubbles_controller
Test: Manual, force-stop com.android.systemui while using the demo
Change-Id: I76ac090689b4ede9821123eea9b22c0ccc678775