SearchManager: Lazy build the searchable list.

- Avoid building searchable list during the user unlocking, only rebuild
  the list when apps request if necessary.
- Invalidate the searchable list when packages containing searchables
  get changed.

Bug: 319129155
Test: atest CtsAppTestCases:android.app.cts.SearchManagerTest
      atest FrameworksServicesTests:com.android.server.search.SearchablesTest

Change-Id: I6afec9258f108e5bd9633740aae9ba3c6b7c88f0