fix(overlays): correctly re-add root to accessibility tree (#28183)

Issue number: resolves #28180

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## What is the current behavior?
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When presenting an overlay, we remove the root (usually
`ion-router-outlet`) from the accessibility tree. This makes it so you
cannot accidentally focus elements behind the overlay. When dismissing
an overlay we re-add the root to the accessibility tree. However, we
fail to consider if there are multiple presented overlays. For example,
if you present a modal, then an alert, then dismiss the alert, then the
root is re-added to the accessibility tree even though the modal is
still presented.

## What is the new behavior?
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this PR. -->

- The root is now re-added to the accessibility tree only if it is the
last presented overlay.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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screenshots of how the component looks before and after the change. -->

Dev build: `7.4.1-dev.11694783260.13da477f`