Input label, hint and placeholder colors don't follow material design spec

[gtranter]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

The label, placeholder, and hint text should be 54% black according to https://material.io/guidelines/components/text-fields.html#text-fields-states.

What is the current behavior?

They are currently 38% black from the 'hint-text' value of the light theme foreground palette.

What are the steps to reproduce?

https://material.angular.io/components/component/input

What is the use-case or motivation for changing an existing behavior?

Follow the guidelines properly. They look too faint.

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

2.0.0-beta.6

Is there anything else we should know?

[angular-automatic-lock-bot]

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