Adds a public Contextual Search invocation method.

- Adds the new startContextualSearch() method.
- Makes ContextualSearchManager non-hidden to expose that method.
- The above clas and method are behind FLAG_SELF_INVOCATION.
- This method requires the caller to have a foreground activity.
- A corresponding startContextualSearchForForegroundApp() method
  is added to the service, which is is non-oneway to propagate
  SecurityException for background calls.

Demo with SupportApp: https://drive.google.com/file/d/1Il-jWGosepboB9462yjn_vGA8hzDXaR2/view?usp=drive_link&resourcekey=0-dvuGjbwlML7UOST3IjJuDw

Bug: 368653769
Test: Build and flash, invoke with SupportApp
Flag: android.app.contextualsearch.flags.self_invocation
Change-Id: I296ce39cb6349e41dc39a20820e5e445860e5c8a