AW: minor flag UI visual improvements

No change to logic. This adds the following minor UX improvements:

* Removes duplicate dividers (one horizontal divider is already built
  into the default Spinner theme), so we have a 1dp divider between
  flags.
* Add back the top-most divider (above the first flag) at the bottom of
  the header. This is now anchored beneath the "reset" button, rather
  than to the top of the first flag.
* Increases vertical padding and margin for the Spinner. This makes it a
  bigger tap-able target.

Fixed: 1059480
Test: Manual - verify Spinner has more space and is easier to tap
Change-Id: I9e492b43c9a1b2c1bc82dbc45b735b7c62c32e9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095923Reviewed-by: Hazem Ashmawy <hazems@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748942}

android_webview/nonembedded/java/res_devui/layout/activity_flags.xml

@@ -38,12 +38,18 @@
         android:id="@+id/reset_flags_button"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
+        android:layout_marginBottom="5dp"
         android:textAppearance="?android:attr/textAppearanceMedium"/>
 
+    <!-- horizontal divider -->
+    <View
+        android:layout_width="match_parent"
+        android:layout_height="1dp"
+        android:background="?android:attr/listDivider"/>
+
     <ListView
         android:id="@+id/flags_list"
         android:layout_width="match_parent"
-        android:layout_height="match_parent"
-        android:layout_marginTop="5dp"/>
+        android:layout_height="match_parent"/>
 
 </LinearLayout>

android_webview/nonembedded/java/res_devui/layout/flag_states.xml

@@ -16,4 +16,7 @@
     android:clickable="false"
     android:textStyle="bold"
     android:textAppearance="?android:attr/textAppearanceSmall"
-    android:padding="2dp" />
+    android:layout_marginTop="5dp"
+    android:layout_marginBottom="5dp"
+    android:paddingTop="10dp"
+    android:paddingBottom="10dp" />

android_webview/nonembedded/java/res_devui/layout/toggleable_flag.xml

@@ -13,12 +13,6 @@
     android:layout_marginTop="15dp"
     android:orientation="vertical">
 
-    <!-- horizontal divider -->
-    <View
-        android:layout_width="match_parent"
-        android:layout_height="1dp"
-        android:background="?android:attr/listDivider"/>
-
     <TextView
         android:id="@+id/flag_name"
         android:layout_width="match_parent"