snapuserd: Make header_response a state variable.

header_response is meant to only be true for the first call to
WriteDmUserPayload. Codify this by making it a member variable and
resetting it on each request.

Bug: 288273605
Test: snapuserd_test
Change-Id: Ic92f5932391a607b63345d579f379d12e78e8f6c