Remove null entries from constant state cache

We insert null entries on cache miss, which can cause NPE on prune, but
we ought to remove these entries during prune.

BUG: 18842826
Change-Id: I9c0bca04f34575d1403de943abd8d12b18c9c032

core/java/android/content/res/Resources.java

@@ -2463,7 +2463,7 @@ public class Resources {
         final int N = cache.size();
         for (int i = N - 1; i >= 0; i--) {
             final WeakReference entry = cache.valueAt(i);
-            if (entry.get() == null) {
+            if (entry == null || entry.get() == null) {
                 cache.removeAt(i);
             }
         }