Completely destroy Launcher's Taskbar.

Just removing it temporarily from the window manager is insufficient.
Taskbar can attempt to recreate itself due to many callbacks being
invoked during tests such as the settings changing. After the tests
finish, the launcher process will restart anyways so taskbar will come
back automatically.

Flag: TEST_ONLY
Test: TaskbarUnitTestRule tests
Bug: 230027385
Change-Id: I18245c224f6d2ffc78d8bf80d30275429074f9b5