Add a lint rule to detect unused formatted resources

[theisenp]

In #649 we changed the way Paraphrase generates FormattedResources, so that instead of generating a reference to all strings with ICU arguments, it generates a reference to all strings full stop. That helps catch some correctness issues, but it also exacerbated a quirk in the way that Paraphrase interacts with lint.

Paraphrase breaks the standard Android UnusedResources lint in a couple ways:

• When checkGeneratedSources is disabled, lint reports false positives because it doesn't see resource references in the generated FormattedResources object.
• When checkGeneratedSources is enabled, lint reports false negatives because every resource is technically referenced in the generated FormattedResources object.

I'm proposing a new UnusedFormattedResources lint rule to address the problem. Basically it's a version of UnusedResources that's Paraphrase-aware:

• It records string declarations
• It counts a reference via FormattedResources as a usage of the underlying string
• If doesn't consider references in FormattedResources itself as sufficient to mark a string as used

I'm sure this implementation is missing some nuances, but it at least seems work when I locally apply it to Cash App, so I think it's a reasonable starting point, and we can iterate as issues arise.