themes: Allow primary and accent colors to be the same palette.

[ErinCoughlan]

Currently, Material throws an warning when primary and accent colors are the same (here). This is not disallowed in the Material spec, so the warning should be removed.

The workaround is to duplicate palettes and change the name:

$mdThemingProvider.definePalette(
      'grey-accent', $mdThemingProvider.extendPalette('grey', {}));
$mdThemingProvider.definePalette(
      'grey-background', $mdThemingProvider.extendPalette('grey', {}));