mediapc CTS: Add java docs for the tests Bug: 179084926 Test: atest android.mediapc.cts Change-Id: Ia88684bed34b0c9f471325b9604d1b86e2cb9547 Merged-In: Ia88684bed34b0c9f471325b9604d1b86e2cb9547 (cherry picked from commit 9019e9a5dc3253088316d678876d209591f7a873)