Explicitly start device lock system service.

We used to relay on the "apex-system-service" tag in the APEX
AndroidManifest.xml, however startApexServices() will not start
the service in case of flattened APEXes.

Bug: 290795765
Test: Service is started
Test: atest CtsDeviceLockTestCases

Change-Id: Ib306a6248d8a0dbd30d822f8e82f4fb4f721aca2