logd: last iterator initialized incorrectly

last should start with mLogElements.end() and be updated as
we iterate to find a matching time entry in the list. Since
it is impossible(sic) for a newer start time to be supplied
than the list, the incorrect iterator initialization should
be inconsequential, but if it ever happens this change will
behave correctly and dump nothing.

Test: gTest liblog-unit-tests, logd-unit-tests and logcat-unit-tests
Bug: 36536248
Bug: 36608728
Change-Id: I96998c4b713258f29d5db2e24a83ae562ddf3420