after receiving panic trigger and locking app, remove from history

This makes the app fully exit, and removes it from the Recent Apps listing.

OpenKeychain/src/main/AndroidManifest.xml

@@ -782,6 +782,9 @@
                 <category android:name="android.intent.category.DEFAULT" />
             </intent-filter>
         </activity>
+        <activity
+            android:name=".ui.ExitActivity"
+            android:theme="@android:style/Theme.NoDisplay" />
 
         <!-- Internal services/content providers (not exported) -->
         <service