Fix getIndex impl to return mIndex

Bug: 183752879
Test: atest FontFamilyUpdateRequestTest
Change-Id: I181ffae1b92775374bc14576095a2c32dba9e888
diff --git a/core/java/android/graphics/fonts/FontFamilyUpdateRequest.java b/core/java/android/graphics/fonts/FontFamilyUpdateRequest.java
index 9290497..bfbcfd8 100644
--- a/core/java/android/graphics/fonts/FontFamilyUpdateRequest.java
+++ b/core/java/android/graphics/fonts/FontFamilyUpdateRequest.java
@@ -275,7 +275,7 @@
          * Returns the index of collection
          */
         public @IntRange(from = 0) int getIndex() {
-            return 0;
+            return mIndex;
         }
     }