[Autofill Assistant] Changed overlay color to be consistent with the default...

[Autofill Assistant] Changed overlay color to be consistent with the default modal dialog overlay color, which currently is black @ 0.5 opacity.

Bug: 806868
Change-Id: I2725d8df984810ac5f39dfc62f0a326614a79bb0
Reviewed-on: https://chromium-review.googlesource.com/c/1317890Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/master@{#606076}

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/black_alpha_24"
+        android:background="@color/black_alpha_65"
         android:gravity="center"
         android:clickable="true"
         android:visibility="gone"