build mediametrics_tests for primary arch only

mediametrics_tests uses some shared libraries that are not populated
in both archs, only in the arch that the mediametrics service uses.
Without this, the build system will happily create a test that won't
run because the previously built system won't have the right abi
libraries included.

Bug: 181773920
Test: build, check resulting tree
Change-Id: Ie9d85f2e3e2b6be04feb8d5f85a63091890b17bb