BBufferedDataIO: Fix Write().

I looked at this code as a reference when writing similar code in another
project, and realized then that this was broken to the point where I
don't think anyone could have actually used it:

1. Writes larger than the buffer should only return the amount written
by this Write() call, not the amount flushed beforehand.

2. fDirty was only set if there were bytes remaining after the first
write to the buffer. So, if you wrote small amounts of data, they
would be silently discarded most of the time!

3. When filling the buffer, we might as well flush simultaneously.
This allows the logic to be consolidated into a loop and remove the
duplicate memcpy. In case of failure, just return the bytes-written
as the write could be retried later.

4. Flush() should always return an error or 0, not bytes written.

While at it, add some basic tests for this class.

Change-Id: I2de01d0b31e3fe22863cef21dd7b0b62ed47121b