Remove WindowContextInfoChangeItem pooling (18/n).

Remove the use of ObjectPool in the creation and management of
WindowContextInfoChangeItem object. Instead of being obtained from the
pool, these objects are now directly instantiated, simplifying their
handling and aligning with the broader removal of the object pooling
mechanism.

Bug: 311089192
Test: atest FrameworksCoreTests:ClientTransactionItemTest
Flag: EXEMPT removing com.android.window.flags.disable_object_pool
Change-Id: I2fc0ab3c129d03d4940b451aadd90b9f14d4d5ec