Add an Android Maps style for OneBusAway

[barbeau]

Summary:

Related to #891, which added more distracting POI icons to the map.

We should examine the new Android Maps styling configurations and determine which makes the most sense to adopt in OBA, with the goals being clear and uncluttered maps for public transportation navigation:
https://developers.google.com/maps/documentation/android-sdk/styling

Steps to reproduce:

1. Go to "Nearby" view and look at map

Expected behavior:

Map should be styled and optimized for public transportation

Observed behavior:

We're using the old default style, which will be removed in August 2018 in favor of a new mandatory style (https://issuetracker.google.com/issues/72780606#comment1), which adds POI icons and other potentially distracting icons and text

Device and Android version:

N/A

[barbeau]

Users have indicated a clear preference over time for not having businesses/POIs included (see #891), so we should remove those.