Correcting Taskbar Icon Layout bounds when Taskbar Divider is present

- Breakout CL part 2
- This is a breakout cl from ag/24272821 to make it more readable and atomic.
- This cl consist of change to correct the icon layout bound calculation when taksbar divider line is present in taskbar view.

Test: Manual, Visual
Bug: 265170176
Flag: ENABLE_TASKBAR_PINNING
Change-Id: I259faf2cc0aa302b83337d104c4820885b925a6a