Styleability Tweaks

Small tweaks to facilitate users of the AAR overriding styles,
dimensions, colors, etc.

- Also wrap some init code in FieldHolder and CardNumHolder in
View.isInEditMode() blocks so that users of the AAR don’t get errors in
the Android Studio editor preview.
- Avoid potential exception when validating an empty card number field
in FieldHolder.
- Set the execute bit on gradlew.