Create TaskContinuityMessenge

Create TaskContinuityMessage, which will be the primary data class
for passing application messages. Corresponding serialization and deserialization
logic has also been added

Flag: android.companion.enable_task_continuity
Test: Added unit tests for TaskContinuityMessage, and ContinuityDeviceConnected
Bug: 400970610

Change-Id: Ic5d80056647fc9a474807d45e143d572bacbafb8