mediav2 CTS: Update AdaptivePlaybackTest

- Buffer allocated to store extracted data needn't be two
  times the file size for video tracks
- Test now runs only for async mode (no real value in testing
  both async and sync mode)

Bug: 181898853
Test: atest CtsMediaV2TestCases:AdaptivePlaybackTest
Change-Id: I806f2dd6fd8ac36cfe0316137635b608c3acf611
1 file changed