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/067-preemptive-unpark/src/Main.java b/test/067-preemptive-unpark/src/Main.java
index a16219e..2c099b9 100644
--- a/test/067-preemptive-unpark/src/Main.java
+++ b/test/067-preemptive-unpark/src/Main.java
@@ -18,6 +18,7 @@
 
         System.out.println("GC'ing");
         System.gc();
+        System.runFinalization();
         System.gc();
 
         System.out.println("Asking thread to park");