[Handoff][1/N] Convert Remaining Messages to Records

This change converts remaining messages to records, allowing us to simplify equality checks. I've updated the corresponding unit tests for clarity.

As part of this change, I've removed the now-unused "currentForegroundTaskId" field. Handoff pivoted to manually comparing last used timestamps, which is more reliable

Test: Updated unit tests
Bug: 400970610
Flag: android.companion.enable_task_continuity
Change-Id: I423e0c10962e9d20de4e2e1f72f7abb32e960566