Storage Refactor: Introduce PbapClientObexClient and others

This change introduces an OBEX client object that no longer has the
responsibility of storing contacts once they're downloaded. Instead, it
funnels contacts back to its owner via a callback.

This object will replace PbapClientConnectionHandler.

Bug: 365626536
Bug: 376461939
Test: atest com.android.bluetooth.pbapclient
Test: m com.android.btservices
Change-Id: If254e644c479d870afe22b9cd574d4f883998c98