Changed overlay color from white @ .9 alpha to black @ .24 alpha.

Bug: 806868
Change-Id: I1abba285ea8d44e64d7eaee15387fd2cd5d43323
Reviewed-on: https://chromium-review.googlesource.com/c/1309786Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/master@{#604310}

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

@@ -16,7 +16,7 @@
         android:id="@+id/overlay"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
-        android:background="@color/white_alpha_90"
+        android:background="@color/black_alpha_24"
         android:gravity="center"
         android:clickable="true"
         android:visibility="gone"