MPEG4Extractor: Keep CTS values positive

Media files with zero STTS delta in video track exists, probably malformed files. If the same
video track has edit shift entry to shift(prepone) video frames, then CTS could get negative.
Since negative CTS values are not meaningful, this CL fixes that by only subtracting the lower
of CTS and edit shift value from CTS.

Bug: 117402420

Test: cts-tradefed run cts-dev -m CtsMediaTestCases --compatibility:module-arg CtsMediaTestCases:
      include-annotation:android.platform.test.annotations.RequiresDevice

Change-Id: I49657457f90e9bbee5fc8bf45235eeb7b0873ab6