Taskbar Pinning Preference Rule fixes.

- Actually evaluate statement in tests.
- Enable aconfig flag for taskbar pinning.
- Allow DisplayController Taskbar mode to change in test harness.
- Make DisplayController think it's in desktop mode when needed.

Flag: TEST_ONLY
Bug: 230027385
Test: TaskbarPinningPreferenceRuleTest

Change-Id: I1c55f73cb5a4bb903cd2d628fa47fbd5e8b1bb86