FTS SearchService with AppSearch

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Jetpack recently gained a new library to perform full-text search: AppSearch.

AppSearch is a high-performance on-device search library for managing locally stored, structured data. It contains APIs for indexing data and retrieving data using full-text search. Applications can use AppSearch to offer custom in-app search capabilities, allowing users to search for content even while offline.

Here's the presentation from Google I/O '21: https://youtu.be/xeQ9faYJktM?t=236

It could be useful to add a new implementation of SearchService using AppSearch to offer blazing-fast and cross-publications search.

Here's some ideas on the design of such feature:

• A new FtsSession controller object holding an AppSearchSession and providing:
  • A way to index a given Publication. Could be useful for an app to pre-index all the publications in the background.
  • A method creating a new SearchServiceFactory which will interface a Publication and the FTS database.
• If the publication was not already indexed, it should be indexed on-the-fly during the first search query.
• Rely on the ContentResourceExtractor to get the text content out of any type of resource from the Publication. See StringSearchService for an example.