stagefright: Correct timescale in MvHd for HFR

Either timescale or duration must be modified to show the correct
HFR clip duration. Since duration is also queried by the record
timer, modify timescale instead.

CRs-Fixed: 662909
Change-Id: I6b744b4048b7a2d0bb70e2e3a102385a312bf81b