PBAP: Send correct Get response when storage is locked.

Precondition:
1) DUT is in Direct Boot mode, and user has not unlocked.
2) DUT is connected to carkit supporting PBAP

Usecase:
Carkit sends request for get phonebook size or get phonebook.

Expected Result:
DUT should return error as Service unavailable as per BT PBAP Spec,
instead of sending 0 as length of phonebook, or sending xml with
no vcard in above usecase.

Observed behaviour:
DUT sends 0 as length of phonebook, and sends empty xml for phonebook request.

Fix:
During get request from carkit, return OBEX_SERVICE_UNAVAILABLE error if DUT is in
direct boot mode.

Test: Checked if error code is sent out when storage is not accessible in Direct
boot mode.

Bug: 38302549
Change-Id: I906cf7cef5caf650d0bea6bdf0ff71100ecf761a