1092 reduce heap usage to prevent OOM

This is a fix for #1092 

This lowers the LRUCache size for the drawable pool down to it's original size as well as subsampling the images before they are loaded into heap.

On my MotoG before the changes I could easily get heap usage to exceed 85MB causing OOM errors.

Lowering the LRU size reduces the heap consumption to around 60MB on my device.  Subsampling cuts the heap down to ~45MB usage.

Subsampling makes road labels a bit hard to read, so we should increase the font size on the road mapstyling on our mapbox account.