Bug 1860161 - Fade out overlay scrollbars on Android r=emilio,geckoview-reviewers,m_kato

Noticed while working on bug 1859864 that ScrollbarFadeDuration on
Android is 0. It seemed a bit odd because every other platform with
overlay scrollbars has a non-zero fade duration, but I figured that it
was intentional and for power saving/performance/etc.

Looking closer, it seems this is actually a regression from switching
away from the old java scrollbars to gecko scrollbars (bug 1223928).
The old java scrollbars did fade out (bug 704784).

The fade animation makes scrollbars look nicer imo so it would be cool
if we could bring this back.

Chromium uses a duration of 300ms so we should probably match that,
since people are used to it.
https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/widget/compositing/layer_tree_settings.cc;l=68-84;drc=6d4dcee2c0a7d7750037ffb70de6b82385d7ad0e

Differential Revision: https://phabricator.services.mozilla.com/D191503

widget/android/nsLookAndFeel.cpp

@@ -274,7 +274,7 @@ nsresult nsLookAndFeel::NativeGetInt(IntID aID, int32_t& aResult) {
       break;
 
     case IntID::ScrollbarFadeDuration:
-      aResult = 0;
+      aResult = 300;
       break;
 
     case IntID::ScrollButtonLeftMouseButtonAction: