Revert "Revert "Introduce ActivityTransactionItem and reduce null checks""

This reverts commit fd2be2c1.

Reason for revert: Reland the CL

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.

In addition, add some null pointer checks after getActivityClient().
Also fix a typo in performStopActivityInner() and
performDestroyActivity().

Bug: 127877792
Bug: 164982975
Test: atest frameworks/base/core/tests/coretests/src/android/app/servertransaction
Test: atest ActivityThreadTest ActivityThreadClientTest
Test: atest TransactionExecutorTests#testActivityItemNullRecordThrowsException
Test: atest TransactionExecutorTests#testActivityItemExecute
Test: atest CtsAppTestCases:LocalActivityManagerTest
Change-Id: Ida612f1c8af7ecd7a04e69ade461403b8711508a