p13n: limit the number of thumbnails for text queries

Template queries will request for at most 8 thumbnails. We want to limit
the number of thumbnails to 4 for text queries.

Screenshot: http://screen/356jfN7JbTaE66D

BUG=b:354020293
TEST=unit_tests --gtest_filter="*SeaPenFetcherTest*"
TEST=browser_tests --gtest_filter="*SeaPen*"

Change-Id: Ia688a0799793c1f9a8d14172401fd2cba6251bbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5735993
Reviewed-by: Jason Thai <jasontt@chromium.org>
Commit-Queue: Jerry Liu <pzliu@google.com>
Reviewed-by: Thuong Phan <thuongphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1332508}