Allow launch-next-bubble transition for existing bubbles.

This change enables proper transition handling when launching an already
bubbled task. Previously, scenarios for
TYPE_LAUNCH_OR_CONVERT_TO_BUBBLE_FROM_EXISTING_BUBBLE did not account
for existing bubbles. Key modifications include updating
BubbleController to reuse an existing bubble and DefaultMixedHandler to
accept requests for already bubbled tasks. Consequently,
BubbleTransitions and DefaultMixedTransition can now appropriately
manage and animate these transitions. This resolves issues where
launching from a notification with a collapsed bubble did not expand it,
and launching from a notification with an expanded bubble did not switch
to it.

Bug: 407935283
Bug: 414501728
Bug: 416136909
Bug: 389810202
Flag: EXEMPT bug fix
Test: atest WMShellRobolectricTests:BubbleControllerTest
Test: atest WMShellRobolectricTests:BubbleTaskStackListenerTest
Test: atest WMShellMultivalentTestsOnDevice:BubbleControllerTest
Test: atest WMShellMultivalentTestsOnDevice:BubbleTaskStackListenerTest
Test: atest WMShellUnitTests:DefaultMixedHandlerTest
Change-Id: I277082166682478182984b74b0f22b05c3593641