Remove PauseActivityItem object pooling (7/n).

Remove the use of ObjectPool in the creation and management of
PauseActivityItem 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:ObjectPoolTests
Test: atest FrameworksCoreTests:TransactionParcelTests
Flag: EXEMPT removing com.android.window.flags.disable_object_pool
Change-Id: I7a49fa4e28c36d80ca146e46f627e1e29929c969