Fix broken time comparison.

This change fixes a broken check that excludes all recent tombstones
and ANRs and only preserves old ones. Luckily, limit_by_mtime is only
true for flat (non-zipped) bug reports so we haven't seen this too
often in practice.

Test: manual
Bug: 73759506
Change-Id: Ic7856a4a1f5af3b1d5c5becb7bc83d2def4a71ea