android: Switch to AppCompat/Material theme and use custom Toolbar as AppBar

Also includes some whitespace/formatting changes due to the switch to
Android Studio.

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

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