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
4 files changed