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/109-suspend-check/src/Main.java b/test/109-suspend-check/src/Main.java
index d92b9e5..ae10576 100644
--- a/test/109-suspend-check/src/Main.java
+++ b/test/109-suspend-check/src/Main.java
@@ -28,7 +28,7 @@
         doWhileLoop.start();
         garbage.start();
         for (int i = 0; i < TEST_TIME; i++) {
-          System.gc();
+          Runtime.getRuntime().gc();
           System.out.println(".");
           sleep(1000);
         }