Don't use UiThreadTest for Taskbar Unit tests.

This annotation also runs the rule on the main thread, which can lead to
deadlocks with other threads (e.g. loading the model synchronously).

Change-Id: Ib276e9dc322f6f65bd32658e774d6076efb94f2e
Flag: TEST_ONLY
Test: Taskbar Unit Tests
Bug: 230027385