Introduce TestRule for setting Taskbar mode.

This rule is separate from TaskbarUnitTestRule, because mode isn't
relevant to all Taskbar controllers.

To keep MainThreadInitializedObject behavior consistent across
Robolectric and Instrumented environments, context needs to be a SandboxContext.

Test: TaskbarModeRuleTest
Bug: 230027385
Flag: TEST_ONLY
Change-Id: Iae2e3627b9002e13ee6da135113e7b8bcc4e7d47