permit android vector drawables to be used as markers

[mlc]

Does any other open PR do the same thing?

No.

How did you test this PR?

On Android, add a vector drawable resource to the native code. (Here is a sample if you need one.) Then, try to use it as the {{marker}} parameter. Previously, an exception would be thrown; with this PR, it now works:

java.lang.NullPointerException: Attempt to invoke virtual method 'int android.graphics.Bitmap.getWidth()' on a null object reference
    at com.airbnb.android.react.maps.AirMapMarker.getIcon(AirMapMarker.java:330)
    at com.airbnb.android.react.maps.AirMapMarker.createMarkerOptions(AirMapMarker.java:360)
    at com.airbnb.android.react.maps.AirMapMarker.getMarkerOptions(AirMapMarker.java:294)
    at com.airbnb.android.react.maps.AirMapMarker.addToMap(AirMapMarker.java:316)
    at com.airbnb.android.react.maps.AirMapView.addFeature(AirMapView.java:513)
    at com.airbnb.android.react.maps.AirMapManager.addView(AirMapManager.java:361)
    at com.airbnb.android.react.maps.AirMapManager.addView(AirMapManager.java:28)

[alvelig]

LGTM @rborn 🐽