[Autofill Assistant] Fix onboarding padding.

This CL fixes the padding bottom of the onboarding that was broken in
http://crrev/c/1624194.

Change-Id: Id1a01a7495975b0abc7fe852c8221b5516128750
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627359
Auto-Submit: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Reviewed-by: Clemens Arbesser <arbesser@google.com>
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/master@{#662648}

chrome/android/features/autofill_assistant/java/res/layout/autofill_assistant_onboarding.xml

@@ -10,6 +10,7 @@
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:paddingTop="12dp"
+    android:paddingBottom="@dimen/autofill_assistant_bottombar_vertical_spacing"
     android:paddingStart="24dp"
     android:paddingEnd="24dp"
     android:gravity="center_horizontal"