Remove Start/StopActivityItem obj pooling (6/n).

Remove the use of ObjectPool in the creation and management of
StartActivityItem and StopActivityItem objects. 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
Flag: EXEMPT removing com.android.window.flags.disable_object_pool
Change-Id: Icc9ec34fbdca82ad3d321bb59acfbaf73434638d