Handle cases where bubble transitions for newly launched tasks

- When a bubble invocation is done via a path that isn't associated with
  a particular task (ie. via taskbar menu or notification vs app handle),
  we can use a mixed transition to start a new bubble transition.  This
  is necessary also to allow other features to hook into the enter-bubble
  flow, to clean up as well (ie. when entering via split in the future),
  or when handling transitions out of bubbles.

Bug: 396313972
Bug: 408328557
Flag: EXEMPT bugfix
Test: atest WMShellUnitTests
Test: Launch an app, invoke bubble via taskbar for the same app
Change-Id: I346b999e4c49ff3b11efc8523eedc6d98e0b6fc0