Implemented Sign Out as a first PanicKit Response

This closes #204

briar-android/src/org/briarproject/android/panic/ExitActivity.java

@@ -0,0 +1,40 @@
+package org.briarproject.android.panic;
+
+import android.content.Intent;
+import android.os.Build;
+import android.os.Bundle;
+
+import org.briarproject.android.BaseActivity;
+
+public class ExitActivity extends BaseActivity {
+
+	@Override
+	public void onCreate(Bundle savedInstanceState) {
+		super.onCreate(savedInstanceState);
+
+		if (Build.VERSION.SDK_INT >= 21) {
+			finishAndRemoveTask();
+		} else {
+			finish();
+		}
+
+		System.exit(0);
+	}
+
+	public static void exitAndRemoveFromRecentApps(final BaseActivity activity) {
+		activity.runOnUiThread(new Runnable() {
+			@Override
+			public void run() {
+				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);
+			}
+		});
+
+	}
+}