Display login notification + Notification channel config

[aforcier]

Closes #113, enabling the notification shown during login when the app is dismissed (while logging in with username/password). Also fixes a crash when doing so on Android 8.1, which is what made this a more urgent issue than it previously was.

This PR also imports all the latest changes to login library from WPAndroid, which has no behavior changes for us except for the Android 8.0 notification channel support (for reference, this was originally added to the login library in wordpress-mobile/WordPress-Android#7593).

Notification channel concerns

To get this to work, I had to create our first notification channel. These are quite 'sticky', in the sense that you can't easily modify or configure notification channels once they're created.

(more on Android 8.0 notification channels)

With this PR, the notification settings screen for the app (in the system settings) looks like this:

This is aligned with WPAndroid, which also has a 'General' as well as an 'Important' channel. By doing this though, we're betting that we'll keep that channel around, more or less.

If we did decide to remove the channel (and reassign the login notification to a different channel), we can totally do that, but a section will appear in the notification settings (it's cleared if the app data is reset, though):

I believe on earlier Android versions the actual deleted channel names are shown explicitly.

It's worth noting that any changes would presumably be figured out before the MVLP release, so this would only affect beta users who upgraded and never cleared app data.

@folletto what do you make of this approach? We could also opt for a 'Login' channel instead of 'General' if we think we're likely not to want a General channel in the long run.

I'm marking this as not ready for review in the meantime.

[folletto]

Thanks! Shame it crashes otherwise, but it seems using notification channels can be a good thing in general.

As for the approach (btw, thanks for the detailed explanation) I think we can sensibly start with General, and then split out later from general as we get more feedback about the app and we start adding notifications. I guess there is going to be always "something" in general.

Plus, if it's just a matter of clearing app data is not nice, but it's also ok in the long term even if we decide to rename everything. :)

[aforcier]

Thanks @folletto!

I confirmed that the behavior we discussed of changing assigned channels works (it seemed like it should, but you never know):

1. Create 'General' channel, assign login notification to it, run app
2. Add 'Login' channel (keeping 'General' channel), assign login notification to 'Login' channel, run app
3. Login notification works, and notification settings has both 'General' and 'Login'
4. (Bonus) Disable 'General' notifications in settings, login - Login notification is still shown 👍

[aforcier]

Now ready for review @AmandaRiu!

[AmandaRiu]

@aforcier I know it's not directly related to this PR, bu I added a comment regarding the WordPressLoginFlow targetSdkVersion setting. Also not directly related to this PR I opened a ticket for a minor memory leak I found in LoginBaseFormFragment while testing.

Everything else looks great 👌 I tested across the following devices:

• Emulator API 19
• Emulator API P
• LG G5 API 24
• Pixel 2 API 27

[AmandaRiu]

Just to verify: Are we intentionally not targeting 27? For what it's worth I changed this to 27 and tested on APIs: 19, 24, 27, and P. All good.

[aforcier]

It's only 26 because it's coming from the WordPress app and that's still targeting 26. I believe this is ignored and we're actually targeting 27 for the overall Woo app, so it's probably okay to bump it to 27, but we'd have to be careful not to push that upstream and cause issues for the WordPress app, so I ignored it.