[BUG] Cubeviz: Unit Conversion plugin doesn't respect selected function

[rosteen]

Describe the bug
In Cubeviz, if you change from the default max collapse function to sum (or something else) and then use the unit conversion plugin to change the spectral axis units, you can see that the unit-converted spectrum reverts back to the original collapse function. Changing the function in the viewer options for the converted spectrum also doesn't do anything. It would be nice to be able to change units and still see the sum/mean/whatever collapsed spectrum of the cube.

Package versions (please complete the following information):
macOS-10.15.7-x86_64-i386-64bit
Python 3.9.7 (default, Sep 16 2021, 08:50:36)
[Clang 10.0.0 ]
Numpy 1.22.2
astropy 5.0.1
specutils 1.7.0
spectral-cube 0.6.0
pyyaml 6.0
click 8.0.3
asteval 0.9.26
idna 2.10
traitlets 5.1.1
bqplot 0.12.33
bqplot-image-gl 1.4.6
glue-core 1.2.4
glue-jupyter 0.11.1
glue-astronomy 0.3.2
echo 0.6
ipyvue 1.7.0
ipyvuetify 1.8.2
ipysplitpanes 0.2.0
ipygoldenlayout 0.4.0
voila 0.3.1
vispy 0.9.6
Jdaviz 2.3.1.dev89+g80324766.d20220322