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/074-gc-thrash/src/Main.java b/test/074-gc-thrash/src/Main.java
index f85aa4b..78413f3 100644
--- a/test/074-gc-thrash/src/Main.java
+++ b/test/074-gc-thrash/src/Main.java
@@ -232,7 +232,7 @@
         for (int i = 0; i < MAX_DEPTH; i++)
             strong[i] = null;
 
-        System.gc();
+        Runtime.getRuntime().gc();
 
         for (int i = 0; i < MAX_DEPTH; i++) {
             if (weak[i].get() != null) {