fix: alerts could freeze the application

[dpogue]

Platforms affected

iOS

Motivation and Context

Closes GH-1120.
Closes GH-1121.
Closes GH-1429.

Description

In cases where the system was presenting something on the screen (such as a permission prompt, or an undo dialog) or in cases where the CDVViewController was not the root view controller (such as in a tabbed navigator), trying to show any dialogs triggered from the WebView would attempt to create an alert on the root view controller and get the view hierarchy into a bad state.

Now, we only try to display alerts on the CDVViewController that owns the WebView. This means that alerts can potentially end up behind other system dialogs, but the view controller no longer freezes.

(As an aside, having access to the CDVViewController here will be helpful for some future things too...)

Testing

Tested manually by triggering an alert while a permission prompt was being displayed and confirmed that the app continued working as expected.

Checklist

• I've run the tests to see all new and existing tests pass
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.22%. Comparing base (8e59e52) to head (9cb8f8d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1437   +/-   ##
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  Coverage   80.22%   80.22%           
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  Files          16       16           
  Lines        1871     1871           
=======================================
  Hits         1501     1501           
  Misses        370      370           

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[jcesarmobile]

This will still cause issues like not allowing to stack multiple alerts or not showing them if a view controller is present (i.e. InAppBrowser or Camera).
In Dialogs plugin we have a helper function that gets the top view controller, but not sure if it could cause the freezing issue mentioned here.
https://github.com/apache/cordova-plugin-dialogs/blob/master/src/ios/CDVNotification.m#L151-L161

[dpogue]

I did some testing of alert dialogs with this PR while the camera and file pickers were open, and it seemed to work as expected with no freezes.

[erisu]

LGTM

I retested the new changes, and they appear to be working as expected.

For example:

• If the camera is open and an alert dialog appears, the user must close the dialog before interacting with or closing the camera capture window.
• When the camera capture window is closed, the app does not freeze and continues to operate as expected.
• Calling multiple system dialogs will not render stack; instead, each dialog will display sequentially, after the previous one is closed.