PBAP Download Contacts in state machine

Add refactored code to the new PBAP state machine to implement the safe
downloading of contacts.  Key improvements are no internal state
handling, no complex message passing, and an improved ability to abort
and handle error conditions.
bug: 28249138

Change-Id: I30aa6ab6730d92e3ec797392ee9a1be7f55fa46a
(cherry picked from commit 8bc55a65)