[Handoff][2/N] Filter Associations in TaskContinuityManager to those with Handoff enabled.

If Handoff is disabled for an association, ignore it and do not use the provided transport

Flag: android.companion.enable_task_continuity
Bug: 400970610
Test: Updated Unit Tests
Change-Id: Ibd36813e7117d71707b0597274e7d670e4fa1420