Return parsed sample frequency of ADTS header

In SBR mode, the sample frequency will be doubled. But it can't be
bigger than 48000. When calculating the frame duration, half the doubled
sample frequency won't get the original one.

BUG=internal b/29111813
TEST=Play stream on device, media_unittests

Review-Url: https://codereview.chromium.org/2063443002
Cr-Commit-Position: refs/heads/master@{#399993}
7 files changed