Apply dimAmount to background color applied to ModalStack

[Tyler-V]

PR Checklist

• [ ] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• [ ] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• [ ] All existing tests are passing
• [ ] Tests for the changes are included

What is the current behavior?

What is the new behavior?

Fixes/Implements/Closes #25.

[Tyler-V]

Used as:

  showMapLayers() {
    const options = {
      viewContainerRef: this.vcRef,
      animated: false,
      fullscreen: true,
      dimAmount: 0.5,
    };

    return this.modalDialogService.showModal(MapStylesComponent, options);
  }

<ModalStack dismissEnabled="true" class="modal-container" backgroundColor="transparent">
  <StackLayout class="modal">
    <AbsoluteLayout width="100%" height="50" style="border-bottom-color: #DDD; border-bottom-width: 1;">
      <StackLayout orientation="horizontal" width="100%" height="100%">
        <Label text="Map Styles" fontSize="20" fontWeight="bold" width="100%" textAlignment="center" verticalAlignment="center"></Label>
      </StackLayout>
    </AbsoluteLayout>
    <StackLayout class="p-10">
      <StackLayout>
        <Button text="Streets" class="-primary" (tap)="setStyle(MapStyle.MAPBOX_STREETS)"></Button>
        <Button text="Terrain" class="-primary" (tap)="setStyle(MapStyle.OUTDOORS)"></Button>
        <Button text="Satellite" class="-primary" (tap)="setStyle(MapStyle.SATELLITE_STREETS)"></Button>
      </StackLayout>
    </StackLayout>
  </StackLayout>
</ModalStack>

.modal {
  margin: 20;
  margin-top: 35;
  border-radius: 8;
  horizontal-align: center;
  vertical-align: middle;
  background-color: white;
}

.modal-container {
  padding: 25;
  padding-bottom: 10;
}

[Tyler-V]