liblog: fortify event_log_tags test

Robustness fortification.  Deal with gtest failure if event-log-tags
file is updated, adb sync for example, and thus will report
Shared_Dirty instead of Shared_Clear.  Add check for Private_Dirty
and Private_Clean being clear at zero too as a bonus.

Test: gTest liblog-unit-tests --gtest_filter=liblog.event_log_tags
Bug: 31456426
Change-Id: Id912929fdd65499aa532a15653ef387a997a5054