fix(NativeScriptPlatformRef): Destroy lastModuleRef on exitEvent

[m-abs]

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• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/nativescript-angular/blob/master/DevelopmentWorkflow.md#running-the-tests
• Tests for the changes are included. Don't see how

What is the current behavior?

The application module isn't destroyed when the application exitEvent is triggered.
This causes the previous instance of the nativescript-angular app to be running in a sleep state and ngOnDestroy() on it's compoments, services etc. are not called as expected.

What is the new behavior?

Adds an event listener for the exitEvent in the NativeScriptPlatformRef which destroys the lastModuleRef which triggers ngOnDestroy() as expected.

Fixes #923

[vakrilov]

test

[vakrilov]

LGTM - triggered the CI.
Also I think this is more of a fix than a chore so I have renamed the PR title.

[vakrilov]

test

[etabakov]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[m-abs]

Hi @vakrilov,

I just noticed a problem

On Android 9.

1. Launch an app with this patch applied.
2. Go to system settings -> Accessibility -> Font size and change the font size.
3. Go back to the app.

This will have triggered an exit event and tried to destroy the angular application, but it is still running.
Now in a broken state.

You can see the problem here:

1. Checkout https://github.com/m-abs/tns-ng-leak
2. npm install && bash ./patches/apply-patches.sh
3. Follow the steps above.

I print out the app events:

JS: suspend # Leave the app to change the font size
JS: activityPaused
JS: activityStopped
JS: orientationChanged # Font size changed in while I'm in settings
JS: exit # Returned to the app
JS: activityDestroyed
JS: activityCreated
JS: activityStarted
JS: activityResumed
JS: resume
JS: displayed
JS: displayed

[m-abs]

I solved the problem.

The exit event is triggered because the MainActivity.onDestroy() is called as a part of a restart needed to apply the new settings.

I added a test for this case in the exit event handler.

[SvetoslavTsenov]

test