Use std::shared_ptr<const uint8_t> with a custom deleter to
represent MediaSample::data_ instead of std::vector<uint8_t>.
MediaSample::data_ can be shared by multiple MediaSamples and it is
immutable. A new data instance must be created if the clients want to
modify the underlying data. The new data instance can be transferred
to MediaSample using provided MediaSample::TransferData function.
This avoids unnecessary data copying.
Change-Id: Ib59785a9e19d0abb3283179b12eb6779ee922f79
92e1e39868