Introduce ActivityTransactionItem and reduce null checks

This class is used for activity-targeting item to inherit,
which encapsulate null checks of activity client record.
This patch also change signature of ClientTransactionHandler#handle*
and reduces null checks.

Bug: 127877792
Test: atest frameworks/base/core/tests/coretests/src/android/app/servertransaction
Test: atest ActivityThreadTest ActivityThreadClientTest
Test: atest TransactionExecutorTests#testActivityItemNullRecordThrowsException
Test: atest TransactionExecutorTests#testActivityItemExecute

Change-Id: Idc0e9f583fc04e3a9043c28e83475acc65710731