respond to generic panic trigger Intent by clearing passphrases

PanicKit provides a common framework for creating "panic button" apps that
can trigger actions in "panic responder" apps.  In this case, the response
is to clear all the cached passphrases.

OpenKeychain/src/main/AndroidManifest.xml

@@ -772,6 +772,17 @@
             android:name=".ui.HelpActivity"
             android:label="@string/title_help" />
 
+        <activity
+            android:name=".ui.PanicResponderActivity"
+            android:noHistory="true"
+            android:theme="@android:style/Theme.NoDisplay">
+            <intent-filter>
+                <action android:name="info.guardianproject.panic.action.TRIGGER" />
+
+                <category android:name="android.intent.category.DEFAULT" />
+            </intent-filter>
+        </activity>
+
         <!-- Internal services/content providers (not exported) -->
         <service
             android:name=".service.PassphraseCacheService"

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

@@ -0,0 +1,48 @@
+/*
+ * 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;
+
+import org.sufficientlysecure.keychain.service.PassphraseCacheService;
+
+public class PanicResponderActivity extends Activity {
+
+    public static final String PANIC_TRIGGER_ACTION = "info.guardianproject.panic.action.TRIGGER";
+
+    @SuppressLint("NewApi")
+    @Override
+    protected void onCreate(Bundle savedInstanceState) {
+        super.onCreate(savedInstanceState);
+
+        Intent intent = getIntent();
+        if (intent != null && PANIC_TRIGGER_ACTION.equals(intent.getAction())) {
+            PassphraseCacheService.clearCachedPassphrases(this);
+        }
+
+        if (Build.VERSION.SDK_INT >= 21) {
+            finishAndRemoveTask();
+        } else {
+            finish();
+        }
+    }
+}