refactor(drm): resolve content keys in one licence path behind ServerKeySource

Drop null keys from every licence, server and export source, not only Widevine. dl now finds remote sessions and imports through ServerKeySource instead of private fields.

tests/remote/unit/test_remote_service_server_cdm_license.py

@@ -11,7 +11,7 @@ from uuid import UUID
 
 import pytest
 
-from unshackle.core.remote_service import RemoteService
+from tests.remote.key_source import remote_keys
 
 pytestmark = pytest.mark.unit
 
@@ -46,14 +46,10 @@ class _Client:
 
 
 def _service(server_type, answer):
-    svc = RemoteService.__new__(RemoteService)
+    svc = remote_keys(server_cdm_type=server_type)
     svc._track_alias = {}
     svc._title_alias = {}
-    svc._server_cdm = True
-    svc._server_cdm_type = server_type
-    svc.client_licensed = set()
     svc._session_id = "sess"
-    svc.server_vault_keys = {}
     svc.log = logging.getLogger("test-remote-license")
     svc.client = _Client(answer)
     return svc
@@ -86,7 +82,7 @@ def test_response_type_refreshes_the_server_type():
 
     svc.proxy_license(b"", track, "widevine")
 
-    assert svc._server_cdm_type == "playready"
+    assert svc.server_cdm_type == "playready"
 
 
 def test_proxy_mode_keeps_the_callers_type():