nfc(api): Restore behavior from Android T

When Nfc service is not ready, dont fetch NfcManager to avoid getting
into a state where the app keeps getting a bad NfcManager.

Restoring the fix from aosp/1498599.

Bug: 286025476
Change-Id: I4aaa0a87c38eaddccc566c18b75cd1acedc29d75
Test: Manual tests

core/java/android/nfc/NfcAdapter.java

@@ -711,6 +711,7 @@ public final class NfcAdapter {
                 /* Stale sService pointer */
                 if (sIsInitialized) sIsInitialized = false;
             }
+            return null;
         }
         /* Try to initialize the service */
         NfcManager manager = (NfcManager) context.getSystemService(Context.NFC_SERVICE);