[Dev UI] Consolidate disparate user preferences into a UserPreferencesService

[MichaelDoyle]

There are currently 9 different places independently managing localStorage:

span-preference.service.ts
nav-toggle.service.ts
theme-toggle.service.ts
trace.service.ts
context.service.ts
local-runtime.service.ts
experiment.service.ts
background-tasks.service.ts
model-selector.component.ts