fix(react): hardware back button works in dev mode

[liamdebeasi]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
  • Some docs updates need to be made in the ionic-docs repo, in a separate PR. See the contributing guide for details.
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue URL: resolves #26599

The current NavManager implementation adds an ionBackButton listener in the constructor. This allows it to respond to hardware back button presses on Android. When the component is unmounted, the event listener is removed. The event listener only proceeds if this._isMounted is true (i.e. if the component is mounted). If the component is not mounted, the handler returns and no other hardware back button handlers are processed.

In dev mode in React, the NavManager will be created multiple times, but not every instance will be mounted. As a result, there can be instances of NavManager that receive hardware back button events but effectively block any processing since these instances are not mounted. This results in hardware back button behavior being broken when testing an app in development mode.

This behavior does not impact production mode.

What is the new behavior?

• The ionBackButton listener for NavManager is added in the componentDidMount component lifecycle instead of in the constructor
• Removed the this._isMounted flag

Does this introduce a breaking change?

• Yes
• No

Other information

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