logd: statistics add logspan

Logspan down to the millisecond.  Show a percentage if trimmed by
Chatty messages, a subspan from the newest to the newest chatty in
the log buffer.  Sniff stats.add(elem), stats.subtract(elem) and
stats.dropped(elem) to generate the logspan data.

Test: gTest liblog-unit-tests, logd-unit-tests and logcat-unit-tests
Test: manual check Logspan statistics for being in range, added
      temporary internal instrumentation to confirm expectations.
Bug: 37254265
Change-Id: I09c0d9375d5580315543c747b37976f9eeb9e408