[Handoff][5/N] Notify Listeners of Initial Task State on First Registration

This change updates listener logic to automatically send the list of `RemoteTask`s to new listeners upon registration. As a result of this, the unneeded `getRemoteTasks` method has now been removed

Flag: android.companion.enable_task_continuity
Bug: 400970610
Test: Updated Unit Tests
Change-Id: I73d51aa168fe3217ecdf64b0ba6462005cf9bcd6