fix(checkbox): not handling unknown color palette

[crisbeto]

Fixes the checkbox component not falling back to the default color, if the color is undefined. We need to handle this, because ThemePalette allows undefined.

Fixes #18465.

[jelbourn]

LGTM

[alfaproject]

Any hope to get this merged? Just spent hours trying to figure out if the issue was my custom styles and then stumbled upon this...

[jelbourn]

Bumping to P2 because changing the default click action without setting the color leads to a broken checkbox

[jelbourn]

@crisbeto this has a very subtle breakage. If someone uses the provider to set the default color to primary and also has a color input that sets the value to undefined with the intent to use the default color, this now makes the color accent instead of primary. I think for the case when the value is undefined, we should fall back to the default provided into the config rather than always accent.

[crisbeto]

I never considered that @jelbourn, but it sounds like this is broken inside the mixinColor which is responsible for assigning the default when the current color is undefined. I'll submit a separate PR to fix it and the tabindex mixins, because it could break the apps of people that depend on the current behavior.

[crisbeto]

Blocked on #20125.

[andrewseguin]

One more change will be needed - the MDC checkbox doesn't have a get color checking for the defaultColor #20125, which means setting the color after construction will cause the component to forget about falling back to defaultColor.

Either a get color should be added, or we can check in ngOnChanges if color is falsy and set it back to the options color.

[crisbeto]

Updated to add the same behavior to the MDC checkbox.

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