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.

Start by reproducing the icon behavior in the Material icon overview and the listed list, datepicker, form-field, and button examples, then compare the rendered colors with the linked Material guidelines. Done means icons use the guideline defaults unless an explicit primary, accent, or warning color is set, and progress controls use the requested default opacity.

Written by the indexing model from the issue text.