[Handoff][4/N] Add "Internal Error" code for Handoff.

Add an "Internal Error" code for Handoff if an unexpected failure occurred during the handoff flow.

Flag: android.companion.enable_task_continuity
Bug: 400970610
API-Coverage-Bug: 419601871
Test: Updated Unit Tests
Change-Id: I7060541abec2fc8c23628a639ad6c9309419b573