Unit test for PipSurfaceTransactionHelper

- Cleanup the unused code path in pip2.PipSurfaceTransactionHelper
- Add basic unit test cases for pip2.PipSurfaceTransactionHelper

Flag: com.android.wm.shell.enable_pip2
Bug: 376133026
Test: atest WMShellUnitTests:PipSurfaceTransactionHelperTest
Change-Id: Ia938507883ea8d6b2b460a2f33805de880c8217c