android: Switch to AppCompat/Material theme for dialogs

There is no AppCompatProgressDialog class as the use of ProgressDialog
is discouraged (instead progress bars should be placed in the layout directly).
To display the current ProgressDialog instances correctly on systems < 21 we
modify the window background color.

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

@@ -66,7 +66,7 @@
         <activity
             android:name=".ui.TrustedCertificateImportActivity"
             android:label="@string/import_certificate"
-            android:theme="@style/Theme.AppCompat.Dialog" >
+            android:theme="@style/AlertDialogTheme" >
             <intent-filter>
                 <action android:name="android.intent.action.VIEW" />
                 <category android:name="android.intent.category.DEFAULT" />