Fix #7759: Send token view default token

[StephenHeaps]

Summary of Changes

• Send was picking a random token for the currently selected network after the Migrate User Assets from BraveCore to CoreData change because core data returns the models in a non-deterministic order and we were just selecting the first token.
• Logic is updated to now try and select the current networks default token, fallback to BAT, with fallback to the first visible asset when sorted by name.
• Additionally, we are now presenting Select a Token to Send modal when you first open Send view.

This pull request fixes #7759

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()
• New or updated UI has been tested across:
  • Light & dark mode
  • Different size classes (iPhone, landscape, iPad)
  • Different dynamic type sizes

Test Plan:

1. Have at least 2 tokens on a single network as visible assets (ex. SOL & USDC on Solana Mainnet).
2. Open Buy/Swap and change network to the network with 2 or more visible assets, then close Buy/Swap.
3. Open Send.
4. Verify Select Token to Send automatically presents.
5. Without selecting a token, press Cancel or swipe to dismiss.
6. Verify the selected token is the native asset on network.
7. Close Send & repeat test from step 3 a couple times. Verify the same token is selected by default.

Screenshots:

7759.mp4

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).