liblog: simplify fake_log_device

This still fakes the long removed /dev/log devices, whereas it only
needs to print to stderr, so simplify that code.

Use std::mutex now that it is C++ to easy portability concerns.

Use the proper liblog headers for formatting information instead of
hardcoding a copy.

Test: liblog-host unit test
Change-Id: I310a6e7ad939960300eafa729cbfc535c5ced445