Fix InstantiationException for editing (sliders)

Another instantiation issue, but this one wasn't causing a crash,
just breakage; exposure, contrast, etc. sliders were not functional,
because ParametricEditor instantiates its controls using reflection
and proguard is too aggressive about optimizing them out.

Issue: calyxos#2329
Change-Id: I8b9a68878e3a812557ddb915c03efe176e0a17a5