[flexiglass] Access NSSL#mOwnScrollY trough a getter

NSSL#mOwnScrollY is never set in flexiglass, but for the code reader it
is a few extra steps to verify this assumption. Extract this to access
it through a getter, which explicitly returns 0 in flexiglass.

Bug: 360091533
Test: run sysui with, and witout flexiglass
Flag: EXEMPT mechanical refactor
Change-Id: I2853b999c30f8bb08a604db83aa8967990fcc251