Add Vietnamese to localization plugin

[1ec5]

According to this documentation, Mapbox Streets source now (or will soon) support Vietnamese. The localization plugin should localize into Vietnamese like the other languages.

mapbox-plugins-android/plugin-localization/src/main/java/com/mapbox/mapboxsdk/plugins/localization/MapLocale.java

Lines 109 to 110 in cc88baa

	@StringDef( {LOCAL_NAME, ARABIC, CHINESE, SIMPLIFIED_CHINESE, TRADITIONAL_CHINESE, ENGLISH,
	FRENCH, GERMAN, JAPANESE, KOREAN, PORTUGUESE, RUSSIAN, SPANISH})

/ref mapbox/mapbox-gl-native-ios#173
/cc @langsmith