[Android][WebSignin][UI] Add visial feedback when entry is selected

This CL adds visual feedback when an entry is selected in the expanded
account list.

Bug: 1135141
Change-Id: Idba767ae72a7e9d258a35cf3df1440e6cf35c076
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2449989Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Alice Wang <aliceywang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814177}

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

@@ -8,6 +8,7 @@
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
+    android:background="?attr/selectableItemBackground"
     android:drawablePadding="24dp"
     android:paddingStart="16dp"
     android:paddingEnd="8dp"

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

@@ -8,6 +8,7 @@
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
+    android:background="?attr/selectableItemBackground"
     android:drawablePadding="16dp"
     android:paddingStart="8dp"
     android:paddingEnd="8dp"

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

@@ -6,6 +6,7 @@
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
+    android:background="?attr/selectableItemBackground"
     android:gravity="center_vertical"
     android:orientation="horizontal"
     android:padding="8dp">