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/087-gc-after-link/src/Main.java b/test/087-gc-after-link/src/Main.java
index c99a7d5..e0a187a 100644
--- a/test/087-gc-after-link/src/Main.java
+++ b/test/087-gc-after-link/src/Main.java
@@ -170,7 +170,7 @@
                 ite.printStackTrace();
             }
         }
-        System.gc();
+        Runtime.getRuntime().gc();
         System.out.println("GC complete.");
     }
 }