Abstract account type ready logic behind PbapClientAccountManager

This change takes the existing logic that determines when our PBAP
Client account manager framework Account is recognized by the account
manager framework and ready to be interacted with.

This happens when the user unlocks and when our account manager
AuthenticationService for our account type is started and found by
account manager service.

Once these two things are complete, we can begin adding and removing
accounts for our account type without security exceptions.

Centralizing the account management logic provides better code health
and testability, and allows for future patterns where a device can own
it's storage account instead of the service.

Bug: 365626536
Flag: EXEMPT, mechanical refactor with additional test coverage
Test: atest com.android.bluetooth.pbapclient
Test: m com.android.btservices
Change-Id: I020bdc396190d302bd4d26af376c197e14b928af