logd: sum liblog tag messages

As an extension to the duplicate multiple message filtering, special
case liblog tagged event messages to be summed.  This solves the
inefficient and confusing duplicate message report from the DOS attack
detection such as:

liblog: 2
liblog: 2
liblog: 2
liblog: 2
liblog: 3

which would result in:

liblog: 2
chatty: ... expire 2 lines
liblog: 2
liblog: 3

And instead sums them and turns them all into:

liblog: 11

liblog messages should never be subject to chatty conversion.

Test: liblog-benchmarks manually check for coalesced liblog messages
      and make sure they do not turn into chatty messages.
      Instrumented code to capture sum intermediates to be sure.
Bug: 33535908
Change-Id: I3bf03c4bfa36071b578bcd6f62234b409a91184b