Fix OWNERS file of Companion subdirectory in servicetests/

servicetests/.../companion/ has no OWNERS file, but subdirectories of it have OWNERS files set. As a result, we can modify those subdirectories without getting approval from the owner of servicetests/, but not add new subdirectories or files.

This change resolves that issue by adding an OWNERS file to servicetests/.../companion/

Change-Id: I20401d2ab25dd386c32dd5e80d177a7f9b10035e
FLAG: TEST_ONLY