Fixing bulk reads in ByteArray.MyInputStream

This has never worked properly, but it didn't matter until recently
because neither the JDK's nor early Dalvik's DataInputStream was
exercising it. But in 2.0, Dalvik's DataInputStream prefers bulk
reads, which exercises this code when run on-device.

http://code.google.com/p/android/issues/detail?id=8115

The problem was that System.arraycopy call didn't include the start
offset in the call.