Convert ExpiredActivity to XML

Closes #396

briar-android/res/layout/activity_expired.xml

+<?xml version="1.0" encoding="utf-8"?>
+<TextView
+	xmlns:android="http://schemas.android.com/apk/res/android"
+	android:layout_width="match_parent"
+	android:layout_height="match_parent"
+	android:layout_gravity="center"
+	android:gravity="center"
+	android:text="@string/expiry_warning"
+	android:textSize="@dimen/text_size_large"/>
\ No newline at end of file

briar-android/src/org/briarproject/android/ExpiredActivity.java

 package org.briarproject.android;
 
-import static android.view.Gravity.CENTER;
-import static android.view.WindowManager.LayoutParams.FLAG_SECURE;
-import static org.briarproject.android.TestingConstants.PREVENT_SCREENSHOTS;
-import static org.briarproject.android.util.CommonLayoutParams.MATCH_MATCH;
+import android.app.Activity;
+import android.os.Bundle;
 
 import org.briarproject.R;
-import org.briarproject.android.util.LayoutUtils;
 
-import android.app.Activity;
-import android.os.Bundle;
-import android.widget.LinearLayout;
-import android.widget.TextView;
+import static android.view.WindowManager.LayoutParams.FLAG_SECURE;
+import static org.briarproject.android.TestingConstants.PREVENT_SCREENSHOTS;
 
 public class ExpiredActivity extends Activity {
 
@@ -21,19 +16,6 @@ public class ExpiredActivity extends Activity {
 
 		if (PREVENT_SCREENSHOTS) getWindow().addFlags(FLAG_SECURE);
 
-		LinearLayout layout = new LinearLayout(this);
-		layout.setLayoutParams(MATCH_MATCH);
-		layout.setGravity(CENTER);
-
-		int pad = LayoutUtils.getPadding(this);
-
-		TextView warning = new TextView(this);
-		warning.setGravity(CENTER);
-		warning.setTextSize(18);
-		warning.setPadding(pad, pad, pad, pad);
-		warning.setText(R.string.expiry_warning);
-		layout.addView(warning);
-
-		setContentView(layout);
+		setContentView(R.layout.activity_expired);
 	}
 }