Pwa-module: Don't base theme color on loading bar color

[manniL]

What problem does this feature solve?

If not theme_color in the manifest is set, the loading bar color is used (if set). As the loading bar color is mostly not fitting the theme color, this might lead to weird results (like a green nav bar on android when the loading bar is green).

I'd suggest to not base the theme color on the color of the loading bar.

_{This feature request is available on Nuxt community (#c162)}

[Anima-t3d]

Also when setting loading: false, it will generate manifest with "theme_color":false, which results in a console warning: Manifest: property 'theme_color' ignored, type string expected.

[papertokyo]

And if you're using CSS variables for the loading bar color, it doesn't work either.

[pi0]

Fixed by 3.1.0