Use ExePayloadRef for PrimaryPayloadId and SecondaryPayloadId

[drolevar]

I believe this fixes an issue with the custom BootstrapperApplication Id. This property can be anything, for example, "MyBootstrapper", but it's later assigned to PrimaryPayloadId, which is matched against Payload Id here:

    // Find the primary and secondary bootstrapper application payloads.
    for (DWORD i = 0; i < pUserExperience->payloads.cPayloads; ++i)
    {
        BURN_PAYLOAD* pPayload = pUserExperience->payloads.rgPayloads + i;

        if (!pUserExperience->pPrimaryExePayload && CSTR_EQUAL == ::CompareStringOrdinal(pPayload->sczKey, -1, sczPrimaryId, -1, FALSE))
        {
            pUserExperience->pPrimaryExePayload = pPayload;
        }
        else if (fFoundSecondary && !pUserExperience->pSecondaryExePayload && CSTR_EQUAL == ::CompareStringOrdinal(pPayload->sczKey, -1, sczSecondaryId, -1, FALSE))
        {
            pUserExperience->pSecondaryExePayload = pPayload;
        }
    }

While ExePayloadRef is exactly the reference to the exe payload.

[drolevar]

@robmen Please, tell if more stuff has to be done, like unit tests, etc.

[robmen]

Yeah, need the following:

1. Reference an open issue, it'll need to be approved by triage
2. Add a test (unit or integration, whichever is most appropriate... this one should be a unit test) - especially for an issue like this which is a very subtle change