Add a loading state when switching between Filecoin Mainnet to Testnet

[srirambv]

Description

Add a loading state when switching between Filecoin Mainnet to Testnet

Steps to Reproduce

1. Import Filecoin from Ledger
2. Switch to Testnet
3. Shows Mainnet accounts for a few seconds before loading Testnet accounts

Actual result:

No.loading.state.for.Testnet.mov

Expected result:

Add a loading state so user doesn't pick mainnet accounts for testnet

Reproduces how often:

Easy

Brave version (brave://version info)

Brave	1.40.80 Chromium: 102.0.5005.78 (Official Build) beta (64-bit)
Revision	df6dbb5a9fd82af3f567198af2eb5fb4876ef99c-refs/branch-heads/5005_59@{#3}
OS	All

Version/Channel Information:

• Can you reproduce this issue with the current release? NA
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? NA
• Does the issue resolve itself when disabling Brave Rewards? NA
• Is the issue reproducible on the latest version of Chrome? NA

Miscellaneous Information:

cc: @Douglashdaniel @muliswilliam @cypt4

[yrliou]

Removing 1.40.x as testnet will be disabled.

[srirambv]

Verification passed on

Brave	1.49.108 Chromium: 110.0.5481.104 (Official Build) beta (64-bit)
Revision	46de4a7f41979e829b430bc1ee30ef483aa227ac-refs/branch-heads/5481_77@{#19}
OS	Windows 11 Version 22H2 (Build 22621.755)

• Verified steps from brave/brave-core#16859
• Verified loading state is shown when switching between Mainnet and Testnet account when importing FIlecoin accounts

23201.mp4