Put media.metrics service into proper group

specify a group for the media.metrics service.
the unspecified default is inappropriate.

Bug: 65334712
Bug: 65540999
Test: examination of /proc info for metrics
Change-Id: I0e35a2d676586216bcfee672666803b7fa5eae4d
(cherry picked from commit a61dff07)
(cherry picked from commit d162b02a)