respond to generic panic trigger Intent by locking

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 lock the app, if it has been configured
to do so.

As previously discussed in #5341

Closes #5550

//FREEBIE

AndroidManifest.xml

@@ -497,5 +497,13 @@
         </intent-filter>
     </receiver>
 
+    <receiver
+        android:name=".service.PanicResponderListener"
+        android:exported="true">
+        <intent-filter>
+            <action android:name="info.guardianproject.panic.action.TRIGGER" />
+        </intent-filter>
+    </receiver>
+
 </application>
 </manifest>

src/org/thoughtcrime/securesms/service/PanicResponderListener.java

@@ -0,0 +1,28 @@
+package org.thoughtcrime.securesms.service;
+
+import android.content.BroadcastReceiver;
+import android.content.Context;
+import android.content.Intent;
+
+import org.thoughtcrime.securesms.util.TextSecurePreferences;
+
+/**
+ * Respond to a PanicKit trigger Intent by locking the app.  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 lock the app,
+ * if it has been configured to do so via the Signal lock preference. If the
+ * user has not set a passphrase, then the panic trigger intent does nothing.
+ */
+public class PanicResponderListener extends BroadcastReceiver {
+
+  @Override
+  public void onReceive(Context context, Intent intent) {
+    if (intent != null  && !TextSecurePreferences.isPasswordDisabled(context) &&
+        "info.guardianproject.panic.action.TRIGGER".equals(intent.getAction()))
+    {
+      Intent lockIntent = new Intent(context, KeyCachingService.class);
+      lockIntent.setAction(KeyCachingService.CLEAR_KEY_ACTION);
+      context.startService(lockIntent);
+    }
+  }
+}