fix(progressive_rollout): eliminate non-deterministic behavior

[felangel]

Description

• fix(progress_rollout): eliminate non-deterministic behavior
  • previously the groupNumber was nullable and the update behavior would be different depending on the timing of the SharedPreferences read. If the read was fast, the update would use the correct track -- otherwise it would default to stable. This change eliminates that case and makes the groupNumber non-nullable while also reducing the number of times SharedPreferences is read to just a single time when the app is run.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ 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
• 🧪 Tests
• 📝 Documentation
• 🗑️ Chore

[eseidel]

@bryanoltman should probably review since he wrote this, but lgtm.