Switch from osmdroid 4.1 to mapbox-android-sdk 0.3.0

WIP: adding an additional tile source doesn't seem to combine them.  Asked Mapbox support about this, as it may be necessary to make one of the tile sources into an Overlay.

This requires dropping support for Android API level 8, so for mapbox integration, it will need to be 9+.

Also removed the querying of MLS for position info.  That can be added back in a separate activity, as this Map Activity should be used by stumblers to determine where in their area has already been stumbled.  The testing of MLS responses was never clear in the MapActivity.

The "Refresh" button now refreshes the map back to the user's location.  This is useful if they start to pan around the map a bunch and get lost.