Fix warnings from static analysis tool.

1.NULL_RETURNS:
  a. format()
  Null pointer check is added before calling Tag object's method.

Bug: 275291562
Test: Build ok
Change-Id: Idb1462585ad9225e6e026608190a1b4a5de68713

core/java/android/nfc/tech/NdefFormatable.java

@@ -124,6 +124,9 @@ public final class NdefFormatable extends BasicTagTechnology {
         try {
             int serviceHandle = mTag.getServiceHandle();
             INfcTag tagService = mTag.getTagService();
+            if (tagService == null) {
+                throw new IOException();
+            }
             int errorCode = tagService.formatNdef(serviceHandle, MifareClassic.KEY_DEFAULT);
             switch (errorCode) {
                 case ErrorCodes.SUCCESS: