[Offline imagery] Offline download of Earth Engine assets

Simplify process of offline imagery creation and allow and Earth Engine asset to be downloaded offline. The process might work as follows:

* An asset ID and service account credentials will be needed.
* `getPixels` can be called to fetch high-res images, which are sliced into tiles and stored on the device
* Images are downsampled on device as they're downloaded and stored separately.

If we want this to be more broadly applicable this could also be built into its own app (Flutter?) which could download data to a shared location accessible by Ground.