fix(core): use Capacitor safe-area CSS variables on older WebViews (#30865)
Issue number: internal
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## What is the current behavior?
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The safe area variables are only reliant on `env` variables that are
provided by devices.
## What is the new behavior?
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this PR. -->
Capacitor 8 has released [safe area variable
fallbacks](https://capacitorjs.com/docs/apis/system-bars#android-note)
to provide consistent behaviors with older Android devices:
> Due to a [bug](https://issues.chromium.org/issues/40699457) in some
older versions of Android WebView (< 140), correct safe area values are
not available via the safe-area-inset-x CSS env variables. This plugin
will inject the correct inset values into a new CSS variable(s) named
--safe-area-inset-x that you can use as a fallback in your frontend
styles.
- Updated safe area variables to use the fallbacks provided by
Capacitor.
## Does this introduce a breaking change?
- [ ] Yes
- [x] No
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## Other information
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Dev build: `8.7.13-dev.11765920447.1a01ab8b`
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Co-authored-by: Brandy Smith <brandyscarney@users.noreply.github.com>