Fix spark-button: noPadding attribute didn't work when set programmatically (vs. statically)

This worked:
  HTML: `<spark-button noPadding></>`
But this didn't:
  HTML: `<spark-button id="btn"></>`
  Dart: `(querySelector('#btn') as SparkButton).noPadding = true;`

The reason was the half-working support for data-binding in an inline <style> tag in a Polymer custom element's markup: when used around an inline <style> tag, <template if="{{noPadding}}"> worked for statically set noPadding, but failed to see programmatic changes to it, as described above.