Clear TaskbarDividerPopupView callback upon closing

- Breakout CL part 4
- This is a breakout cl from ag/24272821 to make it more readable and atomic.
- This cl consist of clearing callbacks after calling close once, so that we don't run into scenario where user is able to close popup multiple times leading to crash.

Test: Manual, Visual
Bug: 265170176
Flag: ENABLE_TASKBAR_PINNING
Change-Id: Ie4cd95c69a559a17b7c19a64a71ccf6ac20542d7