Uses static top padding for lockscreen notification space

This removes the usage of NSSLC.getTopPadding() to calculate the number of notifications to display, and uses a static value.

+ Make debugging lines for notification available space more clear.

+ Renamed getMinStackScrollerPadding, as it was used only in Lockscreen.

Fixes: 230083553
Test: Enabled debug lines for visual inspection & made sure notifications don't change count when coming back from camera started from lockscreen
Change-Id: I139d25e2bb57257a18f18f6288f3e18b7a1b72d1