Android 11: export address book

Changes in Android 11 made it so address books are no longer shown. This
changes fixes it.

Based on DAVx5's similar fix.

638f9f9264

app/src/main/AndroidManifest.xml

@@ -127,7 +127,7 @@
         <!-- Address book account -->
         <service
             android:name=".syncadapter.NullAuthenticatorService"
-            android:exported="false">
+            android:exported="true">   <!-- Since Android 11, this must be true so that Google Contacts shows the address book accounts -->
             <intent-filter>
                 <action android:name="android.accounts.AccountAuthenticator"/>
             </intent-filter>