Add option to prefetch low-resolution tiles

[1ec5]

#7741 adds mbgl::Map::getPrefetchZoomDelta() and setPrefetchZoomDelta() for controlling how aggressively the map loads lower-resolution tiles. This issue tracks adding a corresponding public API to the Android SDK.

@tmpsantos says the optimal zoom delta is 4. Since this isn’t a value that most developers would be able to come up with on their own, we’d expose a simpler setPrefetchesTiles(boolean) and corresponding getPrefetchesTiles() or something of that nature.

#9438 is the corresponding issue for iOS and macOS.

/cc @tmpsantos @tobrun

[1ec5]

@tmpsantos says the optimal zoom delta is 4.

Apparently this constant is defined in constants.hpp.