Summary

This PR fixes issue #622 where periodic tasks on Android were running at incorrect frequencies when re-registered with different intervals.

Fixes #622

Problem

When developers register a periodic task multiple times with the same unique name but different frequencies (common during development), the original task frequency persists due to the default ExistingPeriodicWorkPolicy.KEEP behavior.

Example scenario:

1. Register task with 15-minute frequency during testing
2. Later register same task with 2-hour frequency
3. Task continues running every 15 minutes instead of 2 hours

Solution

1. Changed default policy from KEEP to UPDATE for periodic tasks

   • UPDATE policy ensures new configurations replace existing ones
   • Less disruptive than REPLACE - doesn't cancel running workers
   • Preserves original timing calculations

2. BREAKING: Separated ExistingWorkPolicy and ExistingPeriodicWorkPolicy enums

   • Mirrors Android's native WorkManager API design
   • Provides better type safety - impossible to use wrong policy type
   • registerPeriodicTask now requires ExistingPeriodicWorkPolicy

Changes

• ✅ Updated default periodic work policy to UPDATE in WorkManagerUtils.kt
• ✅ Created separate ExistingPeriodicWorkPolicy enum in Pigeon API
• ✅ Updated all platform implementations to use correct policy types
• ✅ Added comprehensive documentation with upstream Android links
• ✅ Created example app demo with frequency dropdown and UPDATE policy
• ✅ Updated CHANGELOG.md files for all affected packages

Testing

• All Flutter tests pass (50 tests across all packages)
• All native Android tests pass
• Example app builds and demonstrates the fix
• Code formatting checks pass

Breaking Change

Before:

Workmanager().registerPeriodicTask(
  "task", "task",
  existingWorkPolicy: ExistingWorkPolicy.replace, // Wrong type
);

After:

Workmanager().registerPeriodicTask(
  "task", "task",
  existingWorkPolicy: ExistingPeriodicWorkPolicy.update, // Correct type
);

This is a well-justified breaking change that improves API clarity and prevents developer confusion.

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