[TIMOB-25864] Android: Notifications should use default channel on Android 8 if not assigned

*Summary:* If an Android app targets API Level 26 or higher, notifications will fail to be posted to the status bar on Android 8.0 or higher unless they're assigned a notification channel as documented here... https://docs.appcelerator.com/platform/latest/#!/api/Titanium.Android.NotificationChannel We should modify notifications to auto-assign them the default notification channel if a channel was not already assigned by the developer. This way, when we modify Titanium to auto-target API Level 26 in the future (see ticket: [TIMOB-25852]), notifications will still work. Avoids a breaking change. Note that if the app targets API Level 25 or lower, Android 8 will auto-assign the notification the default channel for us. *Test:*

Set up "tiapp.xml" to target API Level 26 as shown below.

Build and run the below code on an Android 8 device.

Tap on the "Send Notification" button.

Verify that the notification was posted to the status bar.

<ti:app>
    <android xmlns:android="http://schemas.android.com/apk/res/android">
        <manifest>
    		<uses-sdk android:minSdkVersion="16" android:targetSdkVersion="26"/>
        </manifest>
    </android>
</ti:app>

var notificationId = 0;
var window = Ti.UI.createWindow();
var button = Ti.UI.createButton({ title: "Send Notification" });
button.addEventListener("click", function(e) {
	var notification = Ti.Android.createNotification(
	{
		contentTitle: "Content Title",
		contentText: "Content Text",
		contentIntent: Ti.Android.createPendingIntent(
		{
			intent: Ti.Android.createIntent({}),
		}),
	});
	notificationId++;
	Ti.Android.NotificationManager.notify(notificationId, notification);
});
window.add(button);
window.open();

*Recommended Solution:* This should be implemented similar to the following "aps_sdk" library's PR here... https://github.com/appcelerator/aps_sdk/pull/307 *Note:* This is not an issue with our "ti.cloudpush" module. It already assigns a default channel to the received push notification.