liblog: add android_log_logger_list management

Add list of android_log_logger_list entities as a way of
accessing the list of transport contexts from within
one of the transports.  This will enable us to iterate back
to an internal transport context which may house a last index
or signalling semaphore to propagate a wakeup on a blocking
read.

Test: gTest libcutils-tests, logd-unit-tests, liblog-unit-tests,
      logcat-unit-tests
Bug: 27405083
Change-Id: I0a5e793946c020d97ffe10192369998e4ed92a83