java.time.ZoneId is available only from API 26; on Android 6 it causes
NoClassDefFoundError when building NotificationContentEntity. Replaced
ZoneId.systemDefault().id with TimeZone.getDefault().id (Kotlin) and
getID() (Java) wherever we only need the system timezone ID string.
Same IANA ID and behavior on API 26+. No data or semantic change.
Worker/Scheduler still use java.time for date math; full API 23 there
would require ThreeTenABP or Calendar-based logic.