Fix `GetSortedModificationTimes()` to use the `last_modified` time.

The helper was incorrectly using `last_accessed` when it should have
been using `last_modified' in the returned list. This causes the
`BackendParamsManagerTest.OldestEntriesAreRemovedFirst` test to fail on
gLinux Cloudtop. The test compares tails of the returned sorted lists
for equality, and using a list of `last_accessed` times means the lists
are are not equal since the test is actively accessing the files (which
changes their `last_accessed` time slightly) to iterate over them, both
when deleting the oldest files and when building the list of times.

Bug: 377475540
Change-Id: Ia66af9d485f3afad9abe1feef0248a411c59c064
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6902546
Auto-Submit: Roger McFarlane <rogerm@chromium.org>
Commit-Queue: Roger McFarlane <rogerm@chromium.org>
Reviewed-by: Olivier Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1510336}