Rewriting ShortcutHelperViewModelTest to reduce brittleness

Shortcut helper view model test has been updated to use FakeRepositories
to simplify testing. This removes the need to worry about implementation
details and mocking behaviors of Repository dependencies. This is also
groundwork for future tests.

Flag: com.android.systemui.extended_apps_shortcut_category
Test: atest com.android.systemui.keyboard.shortcut
Bug: 405985619

Change-Id: Iec4501574192a95b37b9c8b6118c1c47a5cd00d9