[flexiglass] Remove mScrollable usages

Scrolling in the NSSL is now handled in flexiglass, and this was for the
most part replaced by scrollingEnabled.

Bug: 360091533
Bug: 360106096
Test: manually tested that things look good when scrolling, including
when a notification is pinned
Flag: com.android.systemui.scene_container

Change-Id: I11d4201f1064e18a998c27c001760be2400a02c7