Refactor scheduler logic to handle idle phase in Flutter

[jinyus]

Description

This fixed a bug in the flutter scheduler where it didn't processing effects scheduled before runApp was called.

Type of Change

• [ ] ✨ New feature (non-breaking change which adds functionality)
• [x] 🛠️ Bug fix (non-breaking change which fixes an issue)
• [ ] ❌ Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🧹 Code refactor
• [ ] ✅ Build configuration change
• [ ] 📝 Documentation
• [ ] 🗑️ Chore

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (96fbc9c) 100.00% compared to head (32cd479) 100.00%. Report is 50 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #74 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 34 35 +1 Lines 957 1014 +57 ========================================= + Hits 957 1014 +57 ``` | [Flag](https://app.codecov.io/gh/jinyus/dart_beacon/pull/74/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jinyus) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/jinyus/dart_beacon/pull/74/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jinyus) | `100.00% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jinyus#carryforward-flags-in-the-pull-request-comment) to find out more.

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