Add optional channelId to NotifierBuilder (Android)

[Eyt-Lev]

Currently local notification now goes into the one channel that is configured at initialization (NotificationChannelData.id). Any application that creates multiple channels for notifications on Android has no way to route a particular notification.

This adds an optional channelId to NotifierBuilder:

localNotifier.notify {
    channelId = "my_channel"
    title = "..."
}

If the channel does not exist on the device (or API < 26), we log a warning and fall back to the configured default channel to ensure nothing breaks silently. The behavior remains the same if channelId is left null.

[Eyt-Lev]

#157 also touches this area, with a different approach: it routes via a payload key and defines all channels up front in the config (replacing notificationChannelData with a list).

This PR is additive instead: the builder-level channelId keeps the existing single-channel config untouched and works for apps that create their channels themselves.