Update backlinks logic to use explicit main intent

- Update to build and use an explicit intent when building the main
  launcher intent.
- Explicit intent is required for main launcher intent because not all
  apps use CATEGORY_DEFAULT for their main launcher activity.
- Update tests to ensure the same.
- Also made an unrelated update to UI to better match the mocks.

Bug: 352033030
Test: atest AppClipsViewModelTest AppClipsTest
Flag: com.android.systemui.app_clips_backlinks
Change-Id: I880c0418a69e84607a0e8b95ab6e006d8ad586af