logcat: continue where we left off

Issue introduced as part of new logcatd functionality in
commit 3fa657e7a1cec6bc76615d5c0d86c1c1678fafd8

Faulty logic, add a gTest to confirm.

Bug: 19608716
Change-Id: Ic1b97def25e03e69faae4398a3dff2ff0f88545e