[core] Add line-gradient property

Porting of https://github.com/mapbox/mapbox-gl-js/pull/6303
See the link above for the description of the feature and
its limitations).

Based on patch from @lbud (Lauren Budorick).

platform/android/MapboxGLAndroidSDKTestApp/src/androidTest/java/com/mapbox/mapboxsdk/testapp/style/layer.junit.ejs

@@ -118,7 +118,7 @@ public class <%- camelize(type) %>LayerTest extends BaseActivityTest {
 
 <% } -%>
 <% for (const property of properties) { -%>
-<% if (property.name != 'heatmap-color') { -%>
+<% if (property['property-type'] !== 'color-ramp') { -%>
 <% if (property.transition) { -%>
 
   @Test