DRY: typed preferences facade — one owner for keys, defaults and clamping

[almothafar]

Problem

Preference keys and their defaults are re-stated at every read site. The sharpest case — the warning/critical levels' defaults exist as literals in three places while canonical constants already exist in a fourth:

• NotificationService.java:1141-1142`` — 40 / `20`
• BatteryLevelReceiver.java:90-91`` — 40 / `20`
• MainActivity.java:308-309`` — 20 / `40`
• [BatteryRangeSliderHelper.java:24-26](https://github.com/almothafar/SimpleBatteryNotifier/blob/f4460acc3ba99320c3efe310fb51456af0939ab0/app/src/main/java/com/almothafar/simplebatterynotifier/ui/preference/BatteryRangeSliderHelper.java#L24-L26) — DEFAULT_CRITICAL / DEFAULT_WARNING (used only by the sliders)

If the default ever changes, the alert engine and the UI silently disagree. Beyond this, the getString(R.string._pref_key_…) + inline-default pair is repeated dozens of times across NotificationService, both receivers, both detectors, BatteryRateTracker, TemperatureUtils and the fragments.

Fix

Introduce a small typed facade (e.g. AppPrefs), the single owner of key + default + clamp per setting:

public final class AppPrefs {
    public static int warningLevel(Context ctx) { ... }      // key + default 40
    public static int criticalLevel(Context ctx) { ... }     // key + default 20
    public static boolean vibrateEnabled(Context ctx) { ... }
    public static String chargeNotificationStyle(Context ctx) { ... } // wraps resolveChargeStyle
    public static int drainLimitPph(Context ctx) { ... }      // absorbs clampDrainLimit
    ...
}

• Existing clamp helpers move in or are wrapped: BatteryRateTracker.clampDrainLimit, FastDrainDetector.clampMinutesToMs — the "a corrupt stored value can't defeat the feature" pattern lives in one place.
• Keep the pure clamps testable (they already are).
• Migrate incrementally — start with the duplicated-default offenders above; a full sweep can trail.
• XML-declared defaults (pref_*.xml) should reference the same values where the framework allows, or a comment ties them together.

(Permalinks pinned at f4460ac; grep the symbol names if lines have moved.)

Acceptance criteria

• The 40/20 defaults exist in exactly one place, used by alert engine, receiver, and both sliders
• No raw pref-key reads with inline defaults outside the facade for the migrated settings
• Clamping behavior unchanged and unit-tested
• BatteryRangeSliderHelper.DEFAULT_* either move to the facade or delegate to it