Better property names for converted vector drawables

[jzbrooks]

When vector drawable is converted into an ImageVector via --format iv, the properties should should be unique so that multiple converted vector drawables output to the same kotlin package don't cause a compile issue for a redeclared property.

Currently, if the vector drawable doesn't have an android:name set on the vector tag, the resulting ImageVector property is called vector.

[jzbrooks]

One way to improve this would be to use the vector's filename for the property since vectors must have a 1:1 correspondence with their containing file.

[jzbrooks]

Split from #158