mat-icon color doesn't follow the guidelines

#### Bug, feature request, or proposal:
mat-icon color doesn't follow the [guidelines](https://material.io/guidelines/style/icons.html)

This example added some extra css to partially fixed the problem, it does so only for the white background theme.

#### What is the expected behavior?
All icons should be initialised according to the [guidelines](https://material.io/guidelines/style/icons.html) when color attribute is not explicitly set to "primary", "accent" or "warning".

#### What is the current behavior?
Icons inherit the properties of the container, most of the time black with 87% opacity.

#### What are the steps to reproduce?
https://material.angular.io/components/icon/overview
https://material.angular.io/components/list/examples (folder icon)
https://material.angular.io/components/datepicker/examples (datepicker icon)
https://material.angular.io/components/form-field/overview (prefixes, suffixes: both text and icon)
https://material.angular.io/components/button/examples (basic icon button, basic fab buttons: which should have also white backgrounds)

#### What is the use-case or motivation for changing an existing behavior?
Consistency with the guidelines, which means less css to write afterwards.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
material 5.1.0

#### Is there anything else we should know?
Progress controls should also have a basic color of black with 54% opacity, so that it will work on the basic fab button. Now the default if primary, which in my opinion is wrong.

Comienza reproduciendo el comportamiento de los iconos en la Material icon overview y en los ejemplos list, datepicker, form-field y button enumerados; después, compara los colores renderizados con las directrices de Material enlazadas. Se considera terminado cuando los iconos usan los valores predeterminados de las directrices, salvo que se establezca explícitamente un color primary, accent o warning, y los controles de progreso usan la opacidad predeterminada solicitada.

Escrito por el modelo de indexación a partir del texto del issue.