Fix # Display custom tokens in swap token list and move those with the selected network native token to the top of the list.

[nuo-xu]

This pull request fixes #4981 #4980

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

1. Add few custom tokens to a network that supportsswap
2. Go to swap screen
3. click the From token field or To token field to go to the available token list
   (Native asset of the current selected network and custom tokens should be listed at the top)
   Note: If the token has been selected in one of the From or To fields, this token becomes not available in the other field.

Screenshots:

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue and pull request is assigned to a milestone (should happen at merge time).