[Code health] Consolidate naming of {activate,select,join,selected,active}Survey

The action of selecting a survey for use in the app (`activateSurvey`) and related state (`activeSurvey`). The occurs when the user selects the survey from the list, or when the app is reopened after a survey is already selected.

Rationale:

* "select" collides with the "select survey from a list"; we can reserve this term for this use case.
* "join" might work, but could be confusing as it implies other side effects beyond those to local app state.

@JSunde @scolsen @shobhitagarwal1612 for your input.