logd: add getEventTag command and service

The event log tag service uses /dev/event-log-tags, pstore and
/data/misc/logd/event-log-tags as sticky storage for the invented
log tags.

Test: gTest liblog-unit-tests, logd-unit-tests & logcat-unit-tests
Bug: 31456426
Change-Id: Iacc8f36f4a716d4da8dca78a4a54600ad2a288dd