Define network security configuration.

For now, we use the Android 7/8 defaults which allow cleartext traffic and trust system CAs.

oeffi/AndroidManifest.xml

@@ -57,6 +57,7 @@
         android:allowBackup="true"
         android:icon="@mipmap/ic_oeffi_stations_color_48dp"
         android:label="@string/app_name"
+        android:networkSecurityConfig="@xml/network_security_config"
         android:theme="@style/My.Theme.Light" >
         <activity android:name=".PreferencesActivity" />
         <activity android:name=".AboutActivity" />

oeffi/res/xml/network_security_config.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<network-security-config>
+
+    <base-config cleartextTrafficPermitted="true" >
+        <trust-anchors>
+            <certificates src="system" />
+        </trust-anchors>
+    </base-config>
+
+</network-security-config>