logd: KISS & fix preserve a day

(cherry pick from commit 5921276a)

Code in 833a9b1e was borken,
simpler approach is to simply check last entry (to save a
syscall) minus EXPIRE_HOUR_THRESHOLD. This does make longer logs
less likely to call upon the spam detection than the algorithm
being replaced, but sadly we ended up with a log entry in the
future at the beginning of the logs confounding the previous
algorithm.

Bug: 21555259
Change-Id: I04fad67e95c8496521dbabb73b5f32c19d6a16c2