Merge AssetStream with AssetStreamAdaptor.

Add enums to the constructor for AssetStreamAdaptor to choose the
different behaviors used by the (former) two different classes.

The old clients of AssetStream now get the following features of
AssetStreamAdaptor
- Debugging statements on error.
- The stream is an SkStreamRewindable.
- getLength() returns the correct value, and the old way of getting
  the length (read(NULL, 0)) is no longer implemented, since it is
  no longer used.
- isAtEnd() returns the correct value. ToT Skia makes it pure virtual,
  so some implementation is necessary.

Change-Id: I2a5395914e4f53830aaefee396556459083a1c56