Prepare the mock startTransaction for expand test

We need to make sure we mock the calls on startTx
as well, while returning a proper transaction.

Bug: 376302431
Flag: com.android.wm.shell.enable_pip2
Test: atest WMShellUnitTests:PipExpandAnimatorTest
Change-Id: I9194d2200284101d3927d96f4e8652416bf1053e