Show a 'loading' state indicator for the asset overview UI

**What problem are you trying to solve?**
Tokens from my account seemed to have disappeared while I was switching ETH and token views. This is a problem because for those 1-2 minutes I thought I lost them while they were loading.

**Describe the solution you'd like**
A **loading** state when the data is getting updating on UI. Solution is same as suggested by Christian in issue #3776, just different context.

Start by reviewing issue #3776, then trace the asset overview UI's data-update path to identify where the view switches between ETH and token data. The work is done when users see a loading indicator during updates and can distinguish loading from missing balances.

Written by the indexing model from the issue text.