Add APIs for system settings

[HidenoriMatsubayashi]

Some system settings are fixed at specific values. It is necessary to make these APIs so that the settings can be changed from the upper layer.

 settings.AddMember("alwaysUse24HourFormat", true, allocator);
 settings.AddMember("textScaleFactor", 1.0, allocator);
 settings.AddMember("platformBrightness", "light", allocator);

See: https://github.com/sony/flutter-embedded-linux/blob/master/src/flutter/shell/platform/linux_embedded/flutter_linuxes_engine.cc#L342