logd: address code fragility in last watermarks

Do not make the assumption that if worstPid is set, that the log
buffer id is not LOG_ID_EVENTS or LOG_ID_SECURITY. Add comments
to prevent future over-optimization based on this assumption.

Make sure we reset mLast[id] = begin() when we mark it unset, but
tell optimizer this is an _impossible_ path.

SideEffects: drop two branches in all erase calls, gain an unordered
             find() on an empty list for events and security buffers.
Test: gTest logd-unit-tests, liblog-unit-test & logcat-unit-tests
Bug: 32247044
Change-Id: Ic156ca2253c050c28021cedf48bedaf7bd692c09