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/036-finalizer/src/Main.java b/test/036-finalizer/src/Main.java
index 4bc7d8d..6195aff 100644
--- a/test/036-finalizer/src/Main.java
+++ b/test/036-finalizer/src/Main.java
@@ -93,7 +93,7 @@
 
         /* this will try to collect and finalize ft */
         System.out.println("gc");
-        System.gc();
+        Runtime.getRuntime().gc();
 
         System.out.println("wimp: " + wimpString(wimp));
         System.out.println("finalize");
@@ -106,7 +106,7 @@
         System.out.println("reborn: " + FinalizerTest.mReborn);
         System.out.println("wimp: " + wimpString(wimp));
         System.out.println("reset reborn");
-        System.gc();
+        Runtime.getRuntime().gc();
         FinalizerTest.mReborn = FinalizerTest.mNothing;
         System.out.println("gc + finalize");
         System.gc();