Release strong Fragment references after exec.

When finished with a Runnable transaction in execPendingActions(),
release the strong reference so that Fragment can be GC'ed.

Bug: 3117761
Change-Id: Ica2f50e363be1bd047bf0474b293efeb814b1264

core/java/android/app/FragmentManager.java

@@ -1307,6 +1307,7 @@ final class FragmentManagerImpl extends FragmentManager {
             mExecutingActions = true;
             for (int i=0; i<numActions; i++) {
                 mTmpActions[i].run();
+                mTmpActions[i] = null;
             }
             mExecutingActions = false;
             didSomething = true;