logd: add getEventTag id= command

This is the precursor for "Plan B" recovery when access to
/dev/event-log-tags is blocked to untrusted zones.  Also
deals with mitigating issues with long-lived mappings that
do not update /dev/event-log-tags when dynamically changed.

Test: gTest logd-unit-test --gtest_filter=logd.getEventTag_42
Bug: 31456426
Bug: 35326290
Change-Id: I3db2e73763603727a369da3952c5ab4cf709f901