VideoThumbnail: improve thumbnailTimeUs.

In many cases, apps directly using MMR will select a frame at the
beginning of the video as the video cover. The use of ThumbnailUtils
should also be consistent to avoid inconsistent display of covers
when transferring videos between different apps.

Change-Id: I64c62e9613131a38e80801a2bd73a4a83c7bcdbd
Signed-off-by: guochuang <guochuang@xiaomi.corp-partner.google.com>