mediapc: add precondition for secure decode tests plus minor import optimization Bug: 235353732 Bug: 235400171 Test: atest CtsMediaPerformanceClassTestCases Change-Id: I0691c4c4fa1c335c0aab4d1b6d9e909453328fd3 (cherry picked from commit a3610977612d42902e8bd0c1f14a1d73a949e575) Merged-In: I0691c4c4fa1c335c0aab4d1b6d9e909453328fd3