Fix styles to remove unscoped definitions

[joshuahiggins]

In Chrome 61, there's a deprecation warning related to this repo.

[Deprecation] Styling master document from stylesheets defined in HTML Imports is deprecated, and is planned to be removed in M65, around March 2018. Please refer to https://goo.gl/EGXzpw for possible migration paths.

The code in question is paper-time-picker-dialog-style.html, where a style rule exists outside of the dom-module definition. As such, this style is applied at the document level versus scoped to the module. This behavior will no longer be supported by Chrome in the coming months.