fix(input): correct invalid colors

[willshowell]

Fixes #4651

cc @mmalerba

It was all related to specificity issues, so I decreased the specificity of .mat-input-ripple's background-color and increased it for .mat-input-placeholder's and .mat-placeholder-required's color.

Also, I am unsure as to what the correct behavior for the required asterisk is. Previously, it was accent while floating for all (primary, accent, warn) inputs. It was only warn when invalid and not focused. I chose to ensure it is warn at all times when invalid, but can revert if that's contrary to the spec.

[mmalerba]

can you put each selector on its own line (makes it obvious that there's multiple and not one complex one).

also I think we can't have rules that end with & in our theme files, so maybe:

.mat-input-placeholder {
  color: $input-underline-color-warn;

  &.mat-accent,
  &.mat-float .mat-placeholder-required {
    color: $input-underline-color-warn;
  }
}

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