logd: logcat --clear respect pruneMargin

While a reader is present, consider it a success, and not busy, if a
buffer is pruned down to pruneMargin plus one second of additional
margin of logspan.  If not busy, no need to trigger any mitigations
regarding the readers, or to report any errors.

Side Effects are we no longer mitigate the reader when performing
chatty filtration. This is a positive side effect because we were
getting --wrap wakeups that seemed premature.

Add kickMe() and isBusy() methods to ease maintenance and uniformity
of actions.

Test: gTest liblog-unit-tests, logd-unit-tests & logcat-unit-tests
Test: manual: 'logcat -b all -c' repeat in a loop, at various logging
      load levels, simultaneously 'logcat -b' all in another session.
Bug: 38046067
Change-Id: I3d0c8a2d416a25c45504eda3bfe70b6f6e09ab27