Fix black screen when drag divider to bottom

This issue is caused when divider position is near bottom,
The bounds hight is less than 0 due to our algorithm, if
we do setWindowCrop by this bounds, the surface bounds
status abnormal which show in winscope is full screen.
This situation make dim layer show full screen.

Whether this is Surface bug or not, it can protect it split
side. Fix it by make sure both split bounds width or height
are always larger than 0.

Fix: 197588020
Test: Active stage split, drag divider to bottom then check
Change-Id: I4bcd9ba353432bf852ad4a60b6ca9234c2cd84f1