Fix corner cases in GetPathDisplayTextForSettings()

Also enhanced unit tests.

BUG=b/253122024, b/326123463
TEST=unit_tests --gtest_filter=FileManagerPathUtilTest.GetPathDisplayTextForSetting

Change-Id: I80d1af5c5fedb9f441fc142c3cac08aa4beb6220
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5316541


Auto-Submit: François Degros <fdegros@chromium.org>
Commit-Queue: Wenbo Jie <wenbojie@chromium.org>
Reviewed-by: Wenbo Jie <wenbojie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1263802}