turn off beaming except where I provide an explicit message

(callback), since it's confusing to have the game offer to beam where
all that's being sent is the install URL.  Hmmm -- maybe we do want
that.  Consider....

xwords4/android/XWords4/AndroidManifest.xml

@@ -69,6 +69,10 @@
                android:name=".XWApp"
                >
 
+    <meta-data android:name="android.nfc.disable_beam_default" 
+               android:value="true" 
+               />
+
     <activity android:name="GamesListActivity" 
               android:label="@string/app_name"
               android:launchMode="standard"