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/083-compiler-regressions/src/Main.java b/test/083-compiler-regressions/src/Main.java
index 4d6aca3..a532141 100644
--- a/test/083-compiler-regressions/src/Main.java
+++ b/test/083-compiler-regressions/src/Main.java
@@ -172,7 +172,7 @@
     }
 
     static void b2302318Test() {
-        System.gc();
+        Runtime.getRuntime().gc();
 
         SpinThread slow = new SpinThread(Thread.MIN_PRIORITY);
         SpinThread fast1 = new SpinThread(Thread.NORM_PRIORITY);
@@ -188,7 +188,7 @@
         try {
             Thread.sleep(3000);
         } catch (InterruptedException ie) {/*ignore */}
-        System.gc();
+        Runtime.getRuntime().gc();
 
         System.out.println("b2302318 passes");
     }