Add line#getGeometry and symbol#getGeometry

[m0nac0]

As noted in #111, circle#getGeometry is already implemented, this implements the equivalents for lines and symbols.
Edit: on Android however, due to issue #130, symbols and lines are currently not draggable. Only the last annotation manager enabled in https://github.com/tobrun/flutter-mapbox-gl/blob/2339ab64c64ff75d544385ae16a35b3998dfc1be/android/src/main/java/com/mapbox/mapboxgl/MapboxMapController.java#L324-L326 supports dragging. I have tested this PR by changing the order of the annotation managers in these lines and getting the current position with the code from this PR worked.

Closes #111

[m0nac0]

The CI failed, because pub would always get the platform_interface and web plugins from pub.dev and not use the local versions. I now changed that and the example app now always uses the local versions of these plugins.
I don't have any idea right now why flutter analyze fails, as the builds run fine and flutter analyze also finds no issues on my machine.

[m0nac0]

OK, the issue was that the CI was running flutter analyze in the top level directory, which still references the hosted plugins.
I changed the CI to analyze the example app, referencing the local versions of the plugins.