Evaluate task conditions against the "Other" answer (MultipleChoiceSelection.other_selected)

[rfontanarosa]

see google/ground-platform#2539.

Because "Other" is not a real option (it doesn't have an ID) it is now represented by a bool flag on both the question and the submission:

• Question: Task.MultipleChoiceQuestion.has_other_option
• Submission: Submission...MultipleChoiceResponses.other_selected (+ other_text)

To let conditions key distinguish this, the condition proto now carries a new field:

message MultipleChoiceSelection {
  repeated string option_ids = 1;
  string task_id = 2;
  bool other_selected = 3;  // NEW
}

When other_selected is true, the condition should pass if the data collector selected "Other" on the referenced task. It may be combined with option_ids (SELECT_MULTIPLE) — the condition passes if any listed option is selected or, when other_selected is true, if "Other" is selected.