Split shade motion: improve expansion timing

In this CL the approach is the following:
1 - Define a minimum content height in NSSL for split shade
2 - Use the minimum height when computing Nssl#getEmptyBottomMargin()
3 - Components (Npvc) that calculate expansion fraction already use
    Nssl#getEmptyBottomMargin().

Bug: 213585177
Test: Manually
Change-Id: Id74531a95300ed093edb21c379b57b4ce6836771