Display MCC and MNC in Settings

Prepare for #10

app/src/main/java/com/github/muellerma/prepaidbalance/ui/PreferenceActivity.kt

@@ -108,6 +108,10 @@ class PreferenceActivity : AppCompatActivity() {
                     .show()
                 true
             }
+
+            val providerCodesPref = getPreference("provider_codes")
+            val config = preferenceManager.context.resources.configuration
+            providerCodesPref.summary = "MCC: ${config.mcc}\nMNC: ${config.mnc}"
         }
     }
 }

app/src/main/res/values/strings.xml

@@ -23,4 +23,5 @@
     <string name="clear_current_data_summary">Data older than 6 months is deleted automatically</string>
     <string name="channel_name_error">Error</string>
     <string name="duplicates">Don\'t save duplicate values</string>
+    <string name="debug">Troubleshooting</string>
 </resources>

app/src/main/res/xml/pref_main.xml

@@ -40,4 +40,11 @@
             android:inputType="number"
             android:defaultValue="1.00" />
     </PreferenceCategory>
+    <PreferenceCategory
+        android:title="@string/debug">
+        <Preference
+            android:key="provider_codes"
+            android:enabled="false"
+            android:persistent="false" />
+    </PreferenceCategory>
 </PreferenceScreen>