PIA-1951: Introduce loading state for the subscriptions screen

[kp-juan-docal]

Summary

As per title. It shows the loading state where the subscription details are being fetched.

Sanity Tests

• [x] While in a slow network. Do a clean install. Open the application. Confirm the loading screen, followed by the screen with the populated plans.
• [x] After the above. Kill the process. Open the app. Confirm subscriptions is shown immediately as we are not fetching the plans always.