Update analytics dashboard view loading state

What does this MR do and why?

Update the analytics dashboard view to use a GlSkeletonLoader instead of the current circular spinner so that it matches the dashboard list view loading state.

Note this feature is hidden behind a default off feature flag.

Screenshots or screen recordings

Before	After
Screen_Recording_2023-08-07_at_16.37.15	Screen_Recording_2023-08-07_at_16.36.10

How to set up and validate locally

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard Product Analytics by setting up the feature at Project > Analytics > Analytics dashboards.
3. Verify the onboarding list item shows correctly.
4. Verify the single dashboard view by opening the Audience dashboard.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #421197 (closed)