WebLayer: Mark WebLayer Shell URL bar as unimportant for autofill

This ensures that the autofill framework won't kick off autofill in the
URL bar. The user could still manually trigger autofill by
long-pressing on the URL bar and selecting the autofill prompt that
pops up. That seems not necessary to fix as that flow will not be
possible in the WebLayer embedder. If it were to be fixed, it would
need to be fixed at the level of the shared AutofillActionModeCallback.

Bug: 1083479
Change-Id: I468b7e729139747479b0f5f6a641a3345fc007af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212125Reviewed-by: Mugdha Lakhani <nator@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771715}

weblayer/shell/android/shell_apk/res/layout/shell_browser_controls.xml

@@ -28,6 +28,7 @@
             android:imeOptions="actionGo"
             android:layout_width="match_parent"
             android:layout_height="match_parent"
+            android:importantForAutofill="no"
             android:textSize="15sp"/>
 
         <View