Improve switch default behavior

When setting the position of the compass and attribution button, we need
to assume the platform default gravity for those elements if no explicit
value has been provided. An incorrect default was assumed for the
attribution button (copied from the compass), so that is corrected here.

The methods for setting gravity do not require a default since they are
only called if an explicit gravity value has been provided. Those
default cases are removed here.

#731 (comment)

Author: n8han

android/src/main/java/com/mapbox/mapboxgl/MapboxMapBuilder.java

@@ -118,7 +118,7 @@ public void setMyLocationTrackingMode(int myLocationTrackingMode) {
   public void setMyLocationRenderMode(int myLocationRenderMode) {
     this.myLocationRenderMode = myLocationRenderMode;
   }
-  
+
   public void setLogoViewMargins(int x, int y) {
         options.logoMargins(new int[] {
             (int) x, //left
@@ -134,7 +134,6 @@ public void setCompassGravity(int gravity) {
       case 0:
         options.compassGravity(Gravity.TOP | Gravity.START);
         break;
-      default:
       case 1:
         options.compassGravity(Gravity.TOP | Gravity.END);
         break;
@@ -154,6 +153,8 @@ public void setCompassViewMargins(int x, int y) {
       case Gravity.TOP | Gravity.START:
         options.compassMargins(new int[] {(int) x, (int) y, 0, 0});
         break;
+      // If the application code has not specified gravity, assume the platform
+      // default for the compass which is top-right
       default:
       case Gravity.TOP | Gravity.END:
         options.compassMargins(new int[] {0, (int) y, (int) x, 0});
@@ -173,7 +174,6 @@ public void setAttributionButtonGravity(int gravity) {
       case 0:
         options.attributionGravity(Gravity.TOP | Gravity.START);
         break;
-      default:
       case 1:
         options.attributionGravity(Gravity.TOP | Gravity.END);
         break;
@@ -193,10 +193,12 @@ public void setAttributionButtonMargins(int x, int y) {
       case Gravity.TOP | Gravity.START:
         options.attributionMargins(new int[] {(int) x, (int) y, 0, 0});
         break;
-      default:
       case Gravity.TOP | Gravity.END:
         options.attributionMargins(new int[] {0, (int) y, (int) x, 0});
         break;
+      // If the application code has not specified gravity, assume the platform
+      // default for the attribution button which is bottom left
+      default:
       case Gravity.BOTTOM | Gravity.START:
         options.attributionMargins(new int[] {(int) x, 0, 0, (int) y});
         break;