[Handoff][2/N] Create TaskContinuityMessageSerializer

Simplify message passing logic by creating a common serializer, verses the old approach of wrapping in TaskContinuityMessage

Test: Updated unit tests
Bug: 400970610
Flag: android.companion.enable_task_continuity
Change-Id: Id56ae66914a41467edfe0408e606c33b4c9a1b14