OEM battery-killer mitigation: per-manufacturer guidance + exemption flow (P3)

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Context

Follow-up to the tracking-architecture review (P3). For a continuous tracker, OEM background killers (Xiaomi/MIUI, Samsung OneUI "sleeping apps", Huawei PowerGenie) are a primary cause of dropped tracking, and they sit outside AOSP Doze/App-Standby APIs.

The app currently uses only Settings.ACTION_IGNORE_BATTERY_OPTIMIZATION_SETTINGS via BatteryOptimizationHelper (and deliberately avoids the REQUEST_IGNORE_BATTERY_OPTIMIZATIONS permission for Play caution).

Work items

1. Per-manufacturer guidance UI. Detect Build.MANUFACTURER and deep-link to the correct OEM screen (MIUI autostart, Samsung battery, Huawei protected apps) with concise instructions, following the dontkillmyapp.com patterns. Surface contextually from the tracking screen when a user-initiated session starts.
2. Battery-optimization exemption flow (decision). Evaluate offering ACTION_REQUEST_IGNORE_BATTERY_OPTIMIZATIONS with a rationale on first user-initiated tracking. Google Play permits the REQUEST_IGNORE_BATTERY_OPTIMIZATIONS permission for apps whose core function requires continuous background work (continuous location tracking qualifies). This is a deliberate Play-policy trade-off — needs a decision, not just code.
3. Detect & nudge. Use PowerManager.isIgnoringBatteryOptimizations() (already wired) plus the ApplicationExitInfo reason added in PR feat(tracker): resilient user-session restart + previous-exit classification #222 to detect repeated OEM/SIGKILL exits and prompt the user with OEM guidance.

Notes

minSdk 26 means several legacy OEM tricks (e.g. the Huawei EMUI-4 LocationManagerService wakelock-tag hack) do not apply.

References

Reference: report section P3 (§4); https://dontkillmyapp.com ; Doze battery-optimization exemption policy.