Remove EnterPip/PipState object pooling (11/n).

Remove the use of ObjectPool in the creation and management of
EnterPipRequestedItem and PipStateTransactionItem 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 WmTests:ActivityRecordTests
Flag: EXEMPT removing com.android.window.flags.disable_object_pool
Change-Id: Id04463b36c426a14c508f944744b518d1c537efe