Make valid prop of EnumSettingsOptionInfo optional.

Before, when this component was only being used by NotificationsModal, it was being passed description which was an array of [text, isValid] tuples.

Based on the isValid value, the text statement would be styled as being factual or false.

However, when we use this component elsewhere--for example in ThreadSettingsPrivacyTab, we're not going to have a list of true/false statements. Rather, we're going to have a brief text description.

This diff lets us include that brief text description without the accompanying styling (icon + strikethrough (or absense of strikethrough)).

In subsequent diffs EnumSettingsOption will be refactored further to make things cleaner. Specifically, changing the description type and variable names.

Depends on D4046

1. Manually modify description (array of tuples) such that the second value is undefined/null.
2. Observe that the EnumSettingsOption appears as expected without check/cross icon or any strikethroughs.