Changing SIMPLIFIED_APPEAR_FRACTION flag to true by default

This makes simplified appearFraction calculation in NotificationStackScrollLayout to be default way of calculation.
It's been enabled in teamfood for a few months now so time to enabled it by default

Bug: 259395680
Test: check NSSL behaviour in portrait with regular notifications and with heads-up notification
Change-Id: I5ea5952f4b2d44f2dd40708060de3b10bb39c55b