[Autofill Assistant] Prevents user from interacting with the page.

Bug: 806868
Change-Id: I1006e4cb3e2d77a2a83b2a1a00d5aacd70aba820
Reviewed-on: https://chromium-review.googlesource.com/c/1340269Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Lukasz Suder <lsuder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608992}

chrome/android/java/res_autofill_assistant/layout/autofill_assistant_sheet.xml

@@ -25,6 +25,7 @@
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:layout_gravity="bottom"
+        android:clickable="true"
         android:background="@drawable/autofill_assistant_bottombar_bg"
         android:orientation="vertical">
 

chrome/android/java/res_autofill_assistant/layout/init_screen.xml

@@ -7,6 +7,7 @@
     xmlns:app="http://schemas.android.com/apk/res-auto"
     xmlns:tools="http://schemas.android.com/tools"
     android:id="@+id/init_screen"
+    android:clickable="true"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:layout_marginTop="100dp"