style(progress): don't use hardcoded hues

[willshowell]

The progress bars and progress spinners were defaulting to the 600 and 100 hues. This will use the default hue and the lighter hue, respectively.

Fixes #3935

[jelbourn]

LGTM

[mmalerba]

Seeing this error: Error: argument `$color` of `opacity($color)` must be a color

       Backtrace:
       	src/lib/core/theming/_theming.scss:55, in function `opacity`
       	src/lib/core/theming/_theming.scss:55, in function `if`
       	src/lib/core/theming/_theming.scss:55, in function `mat-color`
       	src/lib/progress-bar/_progress-bar-theme.scss:61, in function `_mat-progress-bar-buffer`
       	src/lib/progress-bar/_progress-bar-theme.scss:37, in mixin `mat-progress-bar-theme`
       	src/lib/core/theming/_all-theme.scss:42, in mixin `angular-material-theme`
        on line 55 of src/lib/core/theming/_theming.scss
>>   $opacity: if($opacity == null, opacity($color), $opacity);
   ---------------------------------^

[willshowell]

@mmalerba Weird, the only way I'm able to replicate that error is by changing line 37 of _progress-bar-theme like this:

-background: #{_mat-progress-bar-buffer($warn, lighter)};
+background: #{_mat-progress-bar-buffer($warn, invalid-val)};

Do you have any more information/suggestions?

[mmalerba]

Ah ok, I looked into it a bit further and it looks like the project with the failures is constructing a theme with invalid palettes. I'll add back merge-ready since there's nothing to do here, we'll just have to work with that team to correct it.

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