Add support for getMetrics to IDrm

Adds support for the new getMetrics call to the IDrm interface and the
DrmHal implementation. The implementation currently returns a dummy
metric collection. Follow-up CLs will flesh out the implementation.

Bug: 64001676

Test: Ran the new unit test created in a related CL; ran Google Play.
Change-Id: Ia6a3af5b59a30ca55267f7e3ba278a510fc8c81e