Update ART tests to handle new System.gc behavior

Changed calls from System.gc to Runtime.getRuntime.gc where it was
necessary.

Required for:
https://android-review.googlesource.com/#/c/80253/

Change-Id: I2b0622585da54229a6248e95d40134b6d18598a9
diff --git a/test/023-many-interfaces/src/ManyInterfaces.java b/test/023-many-interfaces/src/ManyInterfaces.java
index c4cb4ab..d69a490 100644
--- a/test/023-many-interfaces/src/ManyInterfaces.java
+++ b/test/023-many-interfaces/src/ManyInterfaces.java
@@ -200,7 +200,7 @@
          * Clear the heap.  The various classes involved should already
          * be loaded and ready as a result of instantiating ManyInterfaces.
          */
-        System.gc();
+        Runtime.getRuntime().gc();
 
         start = System.nanoTime();
         testIface001(obj, iter);