LogBackend: change the initial log_threshold to DEFAULT

The log levels have always been very confusing (and badly named), but
this was most confusing: if there's a log level called "default", why
is it not the default?

Closes https://github.com/MusicPlayerDaemon/MPD/issues/926

src/LogBackend.cxx

@@ -61,7 +61,7 @@ ToAndroidLogLevel(LogLevel log_level) noexcept
 
 #else
 
-static LogLevel log_threshold = LogLevel::INFO;
+static LogLevel log_threshold = LogLevel::DEFAULT;
 
 static bool enable_timestamp;