Reduce minSdkVersion to 19

[felix-ht]

This package requires android sdk version 20. The Mapbox Gl library for android only requires version 14. https://docs.mapbox.com/android/maps/overview/

Version 20 is especially weird as it is a specific release for Android Wear.

If it would be possible to reduce the min sdk version to 19 this would include the normal 4.4 Kitkat release and thus include quite a bit of extra devices. In numbers this mean that Api level 20 supports 94.1% of all devices while 19 supports 98.1% of all devices.

[tobrun]

From https://api.flutter.dev/flutter/widgets/AndroidView-class.html

AndroidView class

Embeds an Android view in the Widget hierarchy.

Requires Android API level 20 or greater.

#140 could resolve this long term by not relying on Android but for now, not possible.