android: Make certificate import activity properly transparent

src/frontends/android/app/src/main/AndroidManifest.xml

@@ -118,7 +118,7 @@
         <activity
             android:name=".ui.TrustedCertificateImportActivity"
             android:label="@string/import_certificate"
-            android:theme="@style/AlertDialogTheme" >
+            android:theme="@style/TransparentActivity" >
             <intent-filter>
                 <action android:name="android.intent.action.VIEW" />
                 <category android:name="android.intent.category.DEFAULT" />

src/frontends/android/app/src/main/res/values/styles.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <!--
-    Copyright (C) 2012-2016 Tobias Brunner
+    Copyright (C) 2012-2018 Tobias Brunner
     HSR Hochschule fuer Technik Rapperswil
 
     This program is free software; you can redistribute it and/or modify it
@@ -26,4 +26,14 @@
         <item name="colorAccent">@color/accent</item>
     </style>
 
+    <style name="TransparentActivity" parent="Theme.AppCompat.NoActionBar">
+        <item name="android:backgroundDimEnabled">false</item>
+        <item name="android:colorBackgroundCacheHint">@null</item>
+        <item name="android:windowAnimationStyle">@android:style/Animation</item>
+        <item name="android:windowBackground">@android:color/transparent</item>
+        <item name="android:windowIsFloating">true</item>
+        <item name="android:windowIsTranslucent">true</item>
+        <item name="android:windowNoTitle">true</item>
+    </style>
+
 </resources>