Remove ActivityConfigurationChangeItem pool (8/n).

Remove the use of ObjectPool in the creation and management of
ActivityConfigurationChangeItem 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:ActivityThreadTest
Test: atest FrameworksCoreTests:ObjectPoolTests
Test: atest FrameworksCoreTests:TransactionParcelTests
Test: atest WmTests:ActivityRecordTests
Flag: EXEMPT removing com.android.window.flags.disable_object_pool
Change-Id: Ib458941e91cf86500524b0c3a99c5790f89586ba