Make homepage search bar remain persistent when scrolling

- Modify the scroll flag
- Add a background to avoid overlapping

Fix: 218825541
Test: visual
Change-Id: Ice88c1bb324780e72332dd810e654b1d17daafae
Merged-in: Ice88c1bb324780e72332dd810e654b1d17daafae
(cherry picked from commit 7cf0819b41)

res/layout/settings_homepage_app_bar_regular_phone_layout.xml

@@ -19,6 +19,7 @@
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
+    android:background="?android:attr/colorBackground"
     android:orientation="vertical">
 
     <ImageView

res/layout/settings_homepage_app_bar_two_pane_layout.xml

@@ -17,8 +17,11 @@
 
 <LinearLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:androidprv="http://schemas.android.com/apk/prv/res/android"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
+    android:layout_marginBottom="16dp"
+    android:background="?androidprv:attr/colorSurface"
     android:orientation="vertical">
 
     <FrameLayout
@@ -29,7 +32,6 @@
     <LinearLayout
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
-        android:layout_marginBottom="16dp"
         android:orientation="horizontal">
 
         <include layout="@layout/search_bar_two_pane_version"/>

res/layout/settings_homepage_container.xml

@@ -61,7 +61,7 @@
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
             android:orientation="vertical"
-            app:layout_scrollFlags="scroll|enterAlways|enterAlwaysCollapsed">
+            app:layout_scrollFlags="scroll|exitUntilCollapsed">
 
             <include
                 android:id="@+id/homepage_app_bar_regular_phone_view"