[Android][Mfill] Increase fading edge width

The updated specs require a 80dp scroll shadow to give a stronger hint
that the bar is scrollable.

Screenshots before/after are linked in the bug.

Bug: 1124722
Change-Id: I94dffc2b9375816997cae6c8492c21094a487f18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410134Reviewed-by: Ioana Pandele <ioanap@chromium.org>
Commit-Queue: Friedrich [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807455}

chrome/android/features/keyboard_accessory/internal/java/res/values/dimens.xml

@@ -23,7 +23,7 @@
     <dimen name="keyboard_accessory_suggestion_offset">12dp</dimen>
     <dimen name="keyboard_accessory_suggestion_height">48dp</dimen>
     <dimen name="keyboard_accessory_suggestion_icon_size">20dp</dimen>
-    <dimen name="keyboard_accessory_scroll_shadow_width">40dp</dimen>
+    <dimen name="keyboard_accessory_scroll_shadow_width">80dp</dimen>
     <dimen name="keyboard_accessory_bar_item_cc_icon_width">32dp</dimen>
     <dimen name="keyboard_accessory_tab_icon_width">40dp</dimen>
     <dimen name="keyboard_accessory_tab_size">@dimen/keyboard_accessory_height</dimen>