Synchronize window config updates (4/n)

Before, ClientTransaction can only contain one ActivityTransactionItem,
and store the activity token in ClientTransaction.

Now, each ActivityTransactionItem stores its own activity token, so that
it unblocks to have multiple ActivityTransactionItems in one
ClientTransaction.

Bug: 260873529
Test: atest FrameworksCoreTests:ObjectPoolTests
Test: atest FrameworksCoreTests:TransactionExecutorTests
Change-Id: Ida1bfd63fe5b58280fd66c09e3cdecc62be2d990