Fill in frame duration from sample meta

If sample duration is not available, use the average
frame duration of the sequence; if that's not available
default to 30fps.

bug: 120414514
bug: 139815242
test: test some 25fps heif sequence files, the reported
      duration should be 40ms instead of the default 33ms.

Change-Id: I5ddf0a6cbfb44021dee2955badf3f2772586bb1e