Enforce the gridview vertical spacing to be >= 0

In landscape mode on phone, when the viewpager's height is not able to
fit the grid items, the calculated vertical spacing is negative number,
which results in overlapping text

Bug: 361454352
Test: manual. Maxout the display and font size and view the a11yMenu on
phone in landscape mode
Flag: EXEMPT low risk bugfix

Change-Id: I2fea76399ba6f2b2bbb713ab578e00c439dafb04