DeviceIdleService and Background Scan Scheduling

Background

Background scans (app backfill, full sync, index updates) should respect user idle time and power conditions. Idle and battery logic is currently scattered; we are consolidating it into a global service.

Technical Implementation

Unified signals
- powerMonitor.getSystemIdleTime() for system idle duration.
- powerMonitor.isOnBatteryPower() plus battery level queries (reusing existing broadcast logic).
- Activity tracking and task scheduling (aligned with BackgroundTaskService).
Unified decision API
- Outputs isIdle / idleMs / batteryLevel / charging and nextEligibleAt.
- Provides per-task canRun decisions with reason codes.
Task integration
- App indexing backfill and periodic full sync use DeviceIdleService windows.
- File Index and other scans can migrate to the same decision path.
Fallbacks
- If idle/battery signals are unavailable, fall back to time-based scheduling (low priority).

Configuration

Planned settings (user-configurable):

idleThresholdMs
minBatteryPercent
allowWhenCharging
forceAfterHours