libstagefright: Fix incorrect duration calculation

MPEG4Writer uses timescale parameter for calculating duration.
For HFR, divide timescale with HFR ratio factor to play
at 30fps. Added the same in MovieHeader(mvhd) and track
hdeader(tkhd) atom.

REF: CYNGNOS-922
Change-Id: I0ed92ef7a95c4b2b98567949b3b475d85d6bb16f