ImageReader/Writer: refactor and cleanup

Below changes are included:
* Defer the buffer lock to Image#getPlanes call. This will save quite a bit
CPU cycles associated with lock buffer if the application doesn't really
want to access the data.
* Refactor the code: move some common code to some utility class, and use
one unified consumer (BufferItemConsumer) in ImageReader native implementation.
The code refactoring will also make it easier to support non-opaque image
attach/detach.

Bug: 22356918
Bug: 19962027
Change-Id: I4fb865b0ea3deb6650afc64c32a5906f30e8ccbd