Disable fiat values on any chain other than Ethereum mainnet

**Describe the bug**
Users are seeing fiat values that reference an ethereum mainnet value rather than the custom chain they are connected to.

**Steps to reproduce (REQUIRED)**
Connect to xdai or any other network and then view the $ value underneath the token amounts.

**Expected behavior**
The value does not present itself unless it is in fact the value of the tokens on that chain. Also the FIAT as default option should be disabled on custom rpc networks and testnets.

reference support ticket # 60270

Reproduce the issue by connecting to xdai or another custom RPC network and viewing the fiat value beneath token amounts. Trace the fiat display and default-currency behavior from that UI entry point; done when fiat values are hidden unless valid for the connected chain and FIAT is disabled by default on custom RPC networks and testnets.

Written by the indexing model from the issue text.