Only honor passed-in userid value if work profile flag enabled

Previously work profile screenshots from overview would get saved to
work profile even if the feature wasn't enabled (downstream code assumed
that the userId would only be wp if the feature was enabled).

This CL removes that assumption, only saving to a different profile if
the flag is enabled.

Bug: 255971910
Test: Work and personal profile screenshots from overview and key chord
      with the flags enabled and disabled.
Test: atest TakeScreenshotServiceTest
Test: atest ImageExporterTest
Change-Id: If4164373e28b920ef4bea16902ba3455f09b6893