Improve DataSource interface.

1. This adds way to obtain a DataSource which represents a region of
   data contained in the DataSource.
2. This fixes a design bug in "feed" method where the size parameter
   was an int instead of long.
3. This fixes a bug in ByteBufferDataSource where its mSize field was
   a long instead of an int (ByteBuffer's length cannot be more than
   2^31).

Bug: 27461702
Change-Id: Ib0812784beb581f19d2412e667b8bd018f0a3c78