CrashlyticsKey Facets need a better toString() for debugging

[krogrammer]

All the CrashlyticsKey facets (StringState, BooleanState, etc.) lack a custom toString(), which makes them appear like this when logged to console as part of a telemetry event:

Event with Firebase data
com.kroger.telemetry.firebase.StringState@9f7ee04
com.kroger.telemetry.firebase.BooleanState@42dbfed

It would be more useful if these facets printed their key and value, e.g.:

Event with Firebase data
"firebase_string_key": "some value"
"firebase_boolean_key": true