[Handoff][1/N] Standardize Message Passing Logic

This change standardizes calls to `CompanionDeviceManager` behind `TaskContinuityMessenger`, which will allow for a common place to enable and disable message passing.

The next CL in this chain will finish this standardization by decoupling business logic from ConnnectedAssociationStore entirely

Flag: android.companion.enable_task_continuity
Bug: 400970610
Test: Updated Unit Tests
Change-Id: I2939f3e3cd491d49b3c132e97da0c5a1ea12225a