PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.

What is the current behavior?

Right now only standard hex/rgb(a) color values are supported via the RootLayout api, e.g.

getRootLayout()
        .open(view, {
          shadeCover: {
            color: 'rgba(0,0,0,.5)',

What is the new behavior?

This maintains the existing behavior while also supporting linear-gradient as well, e.g.

getRootLayout()
        .open(view, {
          shadeCover: {
            color: 'linear-gradient(to top, #fff, rgba(0, 0, 0, .7))',