Fix response code for successful image upload.

The API spec specifies the response code for a successful image
upload is supposed to be 204. However, currently the response is
201. This commit fixes this, and sets the response code to 204.

Fixes bug 1136480

Change-Id: I7ea3926c2dd86961f2dd9bc975b10799560ef20a
Merge "Fix response code for successful image upload."