Fix NPE on devices that don't have NFC.

We should return null instead of NPE.

Bug: 5622939
Change-Id: I802f4a99ea9f5e73453d5ea30da4e1d9fa279b9c

core/java/android/nfc/NfcAdapter.java

@@ -360,6 +360,10 @@ public final class NfcAdapter {
         /* use getSystemService() instead of just instantiating to take
          * advantage of the context's cached NfcManager & NfcAdapter */
         NfcManager manager = (NfcManager) context.getSystemService(Context.NFC_SERVICE);
+        if (manager == null) {
+            // NFC not available
+            return null;
+        }
         return manager.getDefaultAdapter();
     }