Fix IO.writeFully() on Android 5

Os.write() did not update the ByteBuffer position before Android 6.

A workaround was added by commit
b882322f7371b16acd53677c4a3adbaaed0aef77, which fixed part of the
problem, but the position was still not updated across calls, causing
the wrong chunk to be written.

Refs <https://android.googlesource.com/platform/libcore/+/d9f7e57f5d09b587d8c8d1bd42b895f7de8fbf54%5E%21/>