[MediaProjection][Metrics] define minimal atom

The atom is a subset of the fields defined in the metrics design, to provide a simple set of metrics only needed for launch.

Bug: 235346337
Change-Id: I5c21015e3711b1ae7af269ac25dd0d906f69f2c0
Test: presubmits
Merged-In: I5c21015e3711b1ae7af269ac25dd0d906f69f2c0