Enable backups, if they are encrypted

See: https://github.com/stevesoltys/seedvault/wiki/Known-Issues

app/src/main/AndroidManifest.xml

@@ -30,15 +30,16 @@
 
 	<application
 		android:name="de.grobox.transportr.TransportrApplication"
-		android:allowBackup="false"
+		android:allowBackup="true"
+		android:fullBackupContent="@xml/backup"
 		android:hardwareAccelerated="true"
 		android:icon="@mipmap/ic_launcher"
 		android:label="@string/app_name"
 		android:networkSecurityConfig="@xml/network_security_config"
 		android:roundIcon="@mipmap/ic_launcher_round"
 		android:supportsRtl="true"
 		android:theme="@style/AppTheme"
-		tools:ignore="AllowBackup,GoogleAppIndexingWarning"
+		tools:ignore="GoogleAppIndexingWarning"
 		tools:targetApi="n">
 
 		<activity

app/src/main/res/xml/backup.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<full-backup-content>
+	<include
+		domain="sharedpref"
+		path="."
+		requireFlags="clientSideEncryption" />
+	<include
+		domain="database"
+		path="."
+		requireFlags="clientSideEncryption" />
+	<!-- we do not include file domain, because there is only the offline map tiles which should be excluded due to size  -->
+</full-backup-content>