[NTP OptOut] Make expand icon right aligned on the screen

+ Make expand icon right aligned for the expandable header with 16dp
  right padding

Bug: 821462
Change-Id: I17f24f891013479c3297e062e5e72f04c1257115
Reviewed-on: https://chromium-review.googlesource.com/960925Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Becky Zhou <huayinz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543258}

chrome/android/java/res/layout/new_tab_page_snippets_expandable_header.xml

@@ -14,8 +14,9 @@
 
     <TextView
         android:id="@+id/header_title"
-        android:layout_width="wrap_content"
+        android:layout_width="0dp"
         android:layout_height="wrap_content"
+        android:layout_weight="1"
         android:textAppearance="@style/BlackCaption" />
 
     <org.chromium.chrome.browser.widget.TintedImageView