Fix surfaceInset adjustment.

surfaceInsets are relative and have the same
sign direction for left/top/right/bottom. This means
we want to subtract the left/top insets and add the
right/bottom in order to expand the surface. The current code
is adding the left/right insets before adding them to the right,
while still subtracting from the left. This ends up expanding the
surface by 3*inset pixels instead of 2*inset pixels.

Bug: 28368990
Change-Id: I6495e39283c7d2494c962f89e95672c5f1f6e1cd