Issue/5530 basic notification channels android o

[mzorz]

Fixes #5530

Note: Please note this PR only creates the needed Notification Channels for Android O, but does not care for the other new restrictions on the platform.(# and #) These will be addressed in other PRs.

This PR creates the Notification Channels as required in Android Oreo, and updates all of the instances of NotificationCompat.Builder constructor calls to pass the channel ID.

This PR creates 2 notification channels (named "Important" and "General"), because the 2fa auth notification should have a maximum priority and now the priorities cannot be set on the Notifications themselves but rather belong to the Channels these notifications belong to on Android O+ devices.

To test:
On Android 8+

1. Run the app
2. Now go to the device's Settings screen -> Apps & notifications -> WordPress
3. Tap on "App Notifications"
4. You should see the 2 new channels created under the section labelled "Categories":
   a. Important
   b. General

On Android N and lower:

• Nothing should change.

cc @nbradbury

[nbradbury]

I'm wondering whether we should use GENERAL_IMPORTANT_CHANNEL_ID for both site creation success and failure?

[mzorz]

We could! Just a general comment: this PR does not change the logic we already had in place (i.e. the same importance is kept normal for notifications that were already using the normal priority, and only high for notifications that were already using the high priority).
That said, I don't mind changing it to GENERAL_IMPORTANT_CHANNEL_ID if you feel strongly about it 👍

[nbradbury]

Nope, no strong feelings about that. I'm ready to merge but looks like there are some checkstyle issues causing Travis to fail.

[mzorz]

oops - just fixed that in b55658d and restarted Travis 👍

[nbradbury]

Travis is still failing, but I'm not sure why.

[mzorz]

@aforcier has thrown some light - what was happening is that the login library was not updated. Updated with his enormous help now. Highlights:

• method signature should not be an issue now, as it's been updated
• the new channel ids need be the same as per the hosting application, so here we're using a trick Alex suggested, by having the same declaration of a strings.xml resource entry that is named equally both in the library and the hosting app. The actual value is replaced at compile time by the one in the hosting app notification_channel_normal_id and notification_channel_important_id.
• a new version of the libs/utils library has been pushed to bintray (also Alex did everything here) with version 1.20.2-beta1. This should get back to 1.20.2 when all proves to be working alright.

🤞 and waiting for Travis now

[mzorz]

There's a funny behavior I'm still trying to check @nbradbury:

• when uploading a Post with media in it, each progress update produces the notification sound
• I can change that manually as a user and turn the sound off, but what I really wanted is to keep things as they were, that is, sounds would only be made when the app was in the background (at least that's my perception of how I've been using it).

Let me check a bit and decide whether we should set the sound off for the NORMAL channel

[nbradbury]

Using setAlertOnlyOnce(true) fixed it, and Travis is finally happy - let's