[PiP2] Add PipTaskListenerTest

Added the PipTaskListenerTest for PipTaskListener class.

Classes are optimized for testing in this change
- Added getter/setter for PipTransitionState#mPinnedTaskLeash
- Allow using the mocked PipResizeAnimator in PipTaskListener

Flag: com.android.wm.shell.enable_pip2
Bug: 376133026
Test: atest WMShellUnitTests:PipTaskListenerTest
Change-Id: Id96cf41fd5b7176d49b1c2f4a54a90af24583bb1