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/072-precise-gc/src/Main.java b/test/072-precise-gc/src/Main.java
index e049221..f6cd8b1 100644
--- a/test/072-precise-gc/src/Main.java
+++ b/test/072-precise-gc/src/Main.java
@@ -88,7 +88,7 @@
          * Getting a zero result here isn't conclusive, but it's a strong
          * indicator that precise GC is having an impact.
          */
-        System.gc();
+        Runtime.getRuntime().gc();
 
         for (int i = 0; i < wrefs.length; i++) {
             if (wrefs[i].get() != null)