[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/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/LineLayer.java

@@ -478,6 +478,34 @@ public class LineLayer extends Layer {
     nativeSetLinePatternTransition(options.getDuration(), options.getDelay());
   }
 
+  /**
+   * Get the LineGradient property
+   *
+   * @return property wrapper value around String
+   */
+  @SuppressWarnings("unchecked")
+  public PropertyValue<String> getLineGradient() {
+    checkThread();
+    return (PropertyValue<String>) new PropertyValue("line-gradient", nativeGetLineGradient());
+  }
+
+  /**
+   * Defines a gradient with which to color a line feature. Can only be used with GeoJSON sources that specify `"lineMetrics": true`.
+   *
+   * @return int representation of a rgba string color
+   * @throws RuntimeException thrown if property isn't a value
+   */
+  @ColorInt
+  public int getLineGradientAsInt() {
+    checkThread();
+    PropertyValue<String> value = getLineGradient();
+    if (value.isValue()) {
+      return rgbaToColor(value.getValue());
+    } else {
+      throw new RuntimeException("line-gradient was set as a Function");
+    }
+  }
+
   @Keep
   private native Object nativeGetLineCap();
 
@@ -574,6 +602,9 @@ public class LineLayer extends Layer {
   @Keep
   private native void nativeSetLinePatternTransition(long duration, long delay);
 
+  @Keep
+  private native Object nativeGetLineGradient();
+
   @Override
   @Keep
   protected native void finalize() throws Throwable;

platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/PropertyFactory.java

@@ -394,6 +394,36 @@ public class PropertyFactory {
     return new PaintPropertyValue<>("line-pattern", expression);
   }
 
+  /**
+   * Defines a gradient with which to color a line feature. Can only be used with GeoJSON sources that specify `"lineMetrics": true`.
+   *
+   * @param value a int color value
+   * @return property wrapper around String color
+   */
+  public static PropertyValue<String> lineGradient(@ColorInt int value) {
+    return new PaintPropertyValue<>("line-gradient", colorToRgbaString(value));
+  }
+
+  /**
+   * Defines a gradient with which to color a line feature. Can only be used with GeoJSON sources that specify `"lineMetrics": true`.
+   *
+   * @param value a String value
+   * @return property wrapper around String
+   */
+  public static PropertyValue<String> lineGradient(String value) {
+    return new PaintPropertyValue<>("line-gradient", value);
+  }
+
+  /**
+   * Defines a gradient with which to color a line feature. Can only be used with GeoJSON sources that specify `"lineMetrics": true`.
+   *
+   * @param expression an expression statement
+   * @return property wrapper around an expression statement
+   */
+  public static PropertyValue<Expression> lineGradient(Expression expression) {
+    return new PaintPropertyValue<>("line-gradient", expression);
+  }
+
   /**
    * The opacity at which the icon will be drawn.
    *