Update MCSystemPick to take a StringRef

Updated for both Android and iOS (not used on desktop; Android is a nop)
Merge branch 'refactor-syntax_unicode_button_menu' into refactor-syntax_unicode_mcsystempick

Conflicts:
	engine/src/button.cpp
	engine/src/mbliphonepick.mm
Merge branch 'refactor-syntax_unicode_mcsystempick' into refactor-syntax_unicode