Changed text type for tan input to textPassword (#46)

Corona-Warn-App/src/main/res/values/styles.xml

@@ -237,7 +237,7 @@
         <item name="android:layout_height">@dimen/submission_tan_input_edittext_size</item>
         <item name="android:alpha">0</item>
         <item name="android:background">@null</item>
-        <item name="android:inputType">text</item>
+        <item name="android:inputType">textPassword</item>
         <item name="android:maxLength">@integer/submission_tan_length</item>
         <item name="android:singleLine">true</item>
     </style>