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

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/ui/ExitActivity.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) 2015-2016 Hans-Christoph Steiner <hans@eds.org>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.sufficientlysecure.keychain.ui;
+
+import android.annotation.SuppressLint;
+import android.app.Activity;
+import android.content.Intent;
+import android.os.Build;
+import android.os.Bundle;
+
+public class ExitActivity extends Activity {
+
+    @SuppressLint("NewApi")
+    @Override
+    protected void onCreate(Bundle savedInstanceState) {
+        super.onCreate(savedInstanceState);
+
+        if (Build.VERSION.SDK_INT >= 21) {
+            finishAndRemoveTask();
+        } else {
+            finish();
+        }
+
+        System.exit(0);
+    }
+
+    public static void exitAndRemoveFromRecentApps(Activity activity) {
+        Intent intent = new Intent(activity, ExitActivity.class);
+
+        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK
+                | Intent.FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS
+                | Intent.FLAG_ACTIVITY_CLEAR_TASK
+                | Intent.FLAG_ACTIVITY_NO_ANIMATION);
+
+        activity.startActivity(intent);
+    }
+}

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/ui/PanicResponderActivity.java

@@ -37,6 +37,7 @@ public class PanicResponderActivity extends Activity {
         Intent intent = getIntent();
         if (intent != null && PANIC_TRIGGER_ACTION.equals(intent.getAction())) {
             PassphraseCacheService.clearCachedPassphrases(this);
+            ExitActivity.exitAndRemoveFromRecentApps(this);
         }
 
         if (Build.VERSION.SDK_INT >= 21) {