Improve the logging system

[swift-kim]

Previous discussion: #75 (comment)

• The embedder should properly control its logging level based on the verbose_logging switch (which is enabled by flutter-tizen run --verbose-system-logs).
  • Example:
    

    engine/shell/common/shell.cc

    Lines 228 to 231 in 9f08d18

    	fml::LogSettings log_settings;
    	log_settings.min_log_level =
    	settings.verbose_logging ? fml::LOG_INFO : fml::LOG_ERROR;
    	fml::SetLogSettings(log_settings);

• Remove unnecessary logs in the embedder or make them more informative. We should remove all occurrences of FT_LOGD and use only FT_LOGI/W/E.

[swift-kim]

Fixed by #108. Thanks to @pkosko.