disable css scrollbar and use chromium overlay scrollbar

Edit: currently the MR only disables css scrollbar, ubports/development/core/hybris-support/lxc-android-config!119 is required to enable OverlayScrollbar

according to #357, the css scroll bar theme seems to affect scrolling smoothness

here is a proof of concept of using https://github.com/chromium/chromium/blob/main/ui/native_theme/native_theme_features.cc#L24 in place of the css scrollbar theme, which has a similar look and feel, while has the additional ability to auto hide (the feature is default enabled on android), as well as smoother rendering

here is a side by side comparison, OverlayScrollbar on the left, css scrollbar theme on the right untitled