OMXNodeInstance: Guard mActiveBuffers by mLock

Test: atest CtsMediaTestCases

Bug: 135003556
Bug: 142777721
Change-Id: I02b5c0be94a674fe21884d9faa0c0c9cfbedd5fc
Merged-In: I02b5c0be94a674fe21884d9faa0c0c9cfbedd5fc
(cherry picked from commit 0329492f9e2169ddba5b19f379ddc842db1efdb1)
1 file changed