Search: Add Relevance Sorting

Add the logic for the ***Relevance*** sorting option such that each match to the search query is as follows:
- 2.0 points per instance in title
- 1.0 point per instance in body
- 0.25 points per instance in tag

Notes with identical points will be ordered by most recently modified.

Start by locating the Android search implementation and the code that orders matching notes; the issue defines the scoring weights for title, body, and tags. Verify that equal scores are ordered by most recently modified, then run the existing search tests if available and add coverage for the relevance and tie-breaking behavior.

Written by the indexing model from the issue text.