launcher: add indexing limit to image search.

As part of PnP request, adds a new limit to the image search indexing.
It limits how many images can be processed per user session. There is file scan at the beginning of each user session, so the remaining images can still be handled at the following session.

Bug: b/306283895
Change-Id: I1678f5ddaf4045983b559e81c26e6f8226256e9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5399824


Reviewed-by: Charles Zhao <charleszhao@chromium.org>
Commit-Queue: CJ Huang <chenjih@google.com>
Cr-Commit-Position: refs/heads/main@{#1278872}