Add file search filtering based on extension to FileSearchProvider

This corrects the number of results returned when the caller needs to
filter on the extension. Without this, the caller may only receive 25
results that all need to be filtered out, giving 0 results even though
there could be 25 valid results that are ranked lower.

Bug: b:360229206
Change-Id: I0e7ad0feb0cab31d023e4e8f7c52385bbaf9d940
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5869269
Reviewed-by: Michael Cui <mlcui@google.com>
Reviewed-by: CJ Huang <chenjih@google.com>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1356838}