Refactor BubblesTransitionObserverTest for clarity.

Simplifies BubblesTransitionObserverTest by removing redundant mock
setups and improving test readability. Common guard condition mocks are
moved to setUp(), reducing duplication across tests. Task transition
creation is streamlined by inlining createTaskInfo() into
createTaskTransition(), which now initializes token to support direct
use of TransitionInfoBuilder#addChange. Adds coverage for cases where a
new task is launched by another bubble. Tests are also migrated to use
TestParameterInjector for clearer, parameterized testing of transition
modes.

Bug: 387193964
Test: atest WMShellUnitTests:BubblesTransitionObserverTest
Flag: EXEMPT refactor
Change-Id: I9da2e92f8ffed8dcbe02f2fd250c80d6c91c9034