Leave network selector state as undefined on 'add custom asset' popup

[HEagle18]

Description

Right now the 'add custom asset' page shows up solana first. We want to make the user select a network and only then will the 'add' button be enabled.

Steps to Reproduce

Actual result:

Expected result:

Reproduces how often:

Brave version (brave://version info)

Version/Channel Information:

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

Other Additional Information:

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

Miscellaneous Information:

[srirambv]

Verification passed on

Brave	1.44.95 Chromium: 106.0.5249.40 (Official Build) (64-bit)
Revision	fab1d91915d2722d6339aaa7f4e9ce44f1e9b103-refs/branch-heads/5249@{#442}
OS	Linux

• Verified steps from brave/brave-core#14471
• Verified Solana network is not selected by default on Add custom asset view
• Verified Add button is only enabled when all the required fields are added

23820.mp4

---

Verification passed on

Brave	1.44.95 Chromium: 106.0.5249.40 (Official Build) (64-bit)
Revision	fab1d91915d2722d6339aaa7f4e9ce44f1e9b103-refs/branch-heads/5249@{#442}
OS	Windows 11 Version 21H2 (Build 22000.978)

• Verified steps from brave/brave-core#14471
• Verified Solana network is not selected by default on Add custom asset view
• Verified Add button is only enabled when all the required fields are added

23820.mp4

---

Verification passed on

Brave	1.44.95 Chromium: 106.0.5249.40 (Official Build) (arm64)
Revision	fab1d91915d2722d6339aaa7f4e9ce44f1e9b103-refs/branch-heads/5249@{#442}
OS	macOS Version 12.4 (Build 21F79)

• Verified steps from brave/brave-core#14471
• Verified Solana network is not selected by default on Add custom asset view
• Verified Add button is only enabled when all the required fields are added

23820.mov