liblog: add LOGGER_STDERR frontend

Standalone, this logger provides no end-to-end capability.  Only
provides a writer, no reader transport.  All output goes, logcat-like,
into the stderr stream.  Output can be adjusted with environment
variables ANDROID_PRINTF_LOG and ANDROID_LOG_TAGS.

liblog_*.__android_log_bswrite_and_print___max print fails if a string
member is truncated with "Binary log entry conversion failed" and -1.
We expose the truncated content in the tests and in LOGGER_STDERR.

The purpose of this transport selection is for command-line tools,
providing a means to shunt the logs to be mixed in with the tool's
error stream.

Test: gTest liblog-unit-tests
Bug: 27405083
Change-Id: If344b6e3e67df2dc86ce317cfad8af8e857727b7