Change Taskbar Window height logic for taskbar pinning.

- Breakout CL part 6
- This is a breakout cl from ag/24272821 to make it more readable and atomic.
- This cl makes default taskbar window height to be transient when the taskbar pinning flag is on, so we don't see taskbar window cutoff during animation.

Test: Manual, Visual
Bug: 265170176
Flag: ENABLE_TASKBAR_PINNING
Change-Id: I37e8ad0f2c4d144476828f101019dcacdea7acb2