Introduce CDM AssociationRequestsProcessor

Moving the code for handling incoming AssociationRequests from
CompanionDeviceManagerServices into a separate class -
AssociationRequestsProcessor.
AssociationRequestsProcessor is reponsible for:
- validating the incoming requests
- making a decision whether user confirmation is required
- launching and communicating with the UI (if needed)

Bug: 197933995
Bug: 194100881
Test: make
Change-Id: I8f0694c44b24697db2917a8ab06a6abceff315e3