refactor(android): centralize notification ID management into a single NotificationIds object

[SERDUN]

Problem

Notification IDs are currently scattered across multiple classes with no shared contract:

Location	ID	Type
ActiveCallNotificationBuilder.NOTIFICATION_ID	1	fixed
IncomingCallService.PLACEHOLDER_NOTIFICATION_ID	3	fixed
StandaloneCallService.NOTIFICATION_ID	97	fixed
IncomingCallNotificationBuilder.notificationId(callId)	≥ 1000	dynamic (callId.hashCode)

There is no single place to understand the full ID space, verify there are no collisions, or safely add a new notification type. The MIN_CALL_NOTIFICATION_ID = 1000 threshold that separates fixed IDs from dynamic per-call IDs is defined inside IncomingCallNotificationBuilder — invisible to anyone working in another class.

Proposed solution

Introduce a single NotificationIds object that owns all notification ID constants and the per-call ID derivation:

object NotificationIds {
    const val ACTIVE_CALL = 1
    const val INCOMING_CALL_PLACEHOLDER = 3
    const val STANDALONE_CALL = 97

    // Per-call IDs are dynamically derived and kept above this threshold
    // to guarantee no collision with the fixed IDs above.
    private const val CALL_ID_MIN = 1000

    fun forCall(callId: String): Int =
        (callId.hashCode() and Int.MAX_VALUE).coerceAtLeast(CALL_ID_MIN)
}

All existing constants and notificationId(callId) usages are migrated to reference NotificationIds.

Benefits

• The full notification ID space is visible in one place
• Collisions between fixed and dynamic IDs are impossible to introduce accidentally
• Adding a new notification type requires updating one file, making conflicts obvious
• The CALL_ID_MIN = 1000 boundary is self-documenting in context

Related

• PR fix(android): restore FSI for incoming calls on Android 14+ (WT-1306) #258 introduced MIN_CALL_NOTIFICATION_ID and notificationId(callId) in IncomingCallNotificationBuilder as part of the WT-1306 fix — this refactor is the logical follow-up