[1/n] AppBounds should be nullable in WindowContainerTransition

It should be possible to set null as a value for appCompat
in WindowConfiguration.appBounds because the related
property in WIndowConfiguration can be null.

Flag: EXEMPT Small change
Bug: 387193964
Test: atest WmTests:WindowContainerTransactionTests

Change-Id: I6ac5e3007d9a35234166c01ad8ca3e9bfc643972