perf: Improve KML bitmap caching and fix icon scaling (#609)

* Test caching BitmapDescriptors

* Update import for AndroidX

* Cache scaled BitmapDescriptors

BitmapDescriptorFactory.fromBitmap() copies the Bitmap on every call. Avoid duplicating Bitmaps for each Marker by caching the scaled BitmapDescriptors.

* Clear Bitmap cache when done

* Add doc comments

* Cleanup comments

* Fix marker icon scaling for screen density

* Scale marker from styles

* Reduce Renderer method access scopes

* Consolidate Renderer image caches into class

* Add method behavior details to doc comment

* Use string scale value as cache key

Author: jeffdgr8