Currently the default for protobuf on Android is to silently drop all log
messages. This makes debugging difficult because things like GOOGLE_LOG(FATAL)
will crash the process without actually logging a useful error.

This CL changes the logging so that by default we send messages for all log
levels to logcat (and stderr). Users can override this by setting
GOOGLE_PROTOBUF_MIN_LOG_LEVEL. Also, that option was not being respected for
non-Android platforms so I went ahead and fixed that to make things consistent.