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/003-omnibus-opcodes/src/Main.java b/test/003-omnibus-opcodes/src/Main.java
index 5dcc79c..25050df 100644
--- a/test/003-omnibus-opcodes/src/Main.java
+++ b/test/003-omnibus-opcodes/src/Main.java
@@ -26,7 +26,7 @@
         main.run();
 
         /* run through the heap to see if we trashed something */
-        System.gc();
+        Runtime.getRuntime().gc();
 
         System.out.println("Done!");
     }