Guard against MountService throwing NPE.

Fall through to below logic and return null instead of crashing the
entire app.  We already tell developers the value may be null.

Bug: 17781998
Change-Id: I05dce90ae6bc547d74f8c16d30b3dc7888a937fe

core/java/android/app/ContextImpl.java

@@ -2419,7 +2419,7 @@ class ContextImpl extends Context {
                         int res = -1;
                         try {
                             res = mount.mkdirs(getPackageName(), dir.getAbsolutePath());
-                        } catch (RemoteException e) {
+                        } catch (Exception ignored) {
                         }
                         if (res != 0) {
                             Log.w(TAG, "Failed to ensure directory: " + dir);