readAt can return negative values (error codes).

This fixes the regression introduced in
59cea261 due to which MediaPlayer
treated all error codes returned by MediaHTTPConnection as
ERROR_OUT_OF_RANGE.

The regression was caused by accidentally converting negative values
(which represent error codes) returned by MediaHTTPConnection to very
large positive ones (which represent length of data received).

Bug: 21922241
Change-Id: I1b4592b5fec724aac1ba6c1ff8fdabcba56bcd2d