fix regression from android_lookupEventTag_len()

Commit 807e40ec 'liblog: logd: Add
android_lookupEventTag_len()' which addressed a Dirty Shared memory
leak resulted in a regression. Most notably logcat <tag> stopped
working for the events log buffer.

AndroidLogEntry::tag also requires callers to check out
AndroidLogEntry::tagLen as tag is no longer guaranteed to be
nul terminated.

Test: logcat-unit-tests --gtest_filter=logcat.event_tag_filter
Bug: 31456426
Change-Id: Ibe5236131b640eb5b7e3df0ab4b5f3e25b85ad45