AbsSeekBar: Fix growRect computation

Fixes an issue with the growRectTo computation where if the
difference between the current and target size wasn't an even
number, the resulting rect would not actually have the correct
size.

Also, asserts using the thumb bounds, rather than the view
bounds in case the thumb is not perfectly centered within the
view due to rounding errors on odd densities.

Fixes: 173872859
Test: atest AbsSeekBarTest
Change-Id: Ib2b81a91ba8feeb6743a4e4a588d6116d231eb8d