[Handoff][7/N] Properly Serialize Application Name

This change fixes how application name is written, by properly reading from `PackageManager`.

As part of this change, logic has been moved out of `RemoteTaskInfo`. I've taken this chance to convert `RemoteTaskInfo` into a record, and adjusted usage accordingly

Test: Validated manually, and updated unit tests with new calls
Bug: 400970610
Flag: android.companion.enable_task_continuity
Change-Id: I8ada40454966cd7313cb914ddb780a34a3db4009