fix(modal): fix enter animation when the initial breakpoint value is less than the backdrop breakpoint (#29955)

Issue number: internal

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## What is the current behavior?
This PR aims to fix an issue introduced in
https://github.com/ionic-team/ionic-framework/pull/29932 where the enter
animation for ionic theme was in a broken state when the modal's initial
breakpoint value was lesser than the specified backdrop breakpoint.

## What is the new behavior?
- The erroneous behaviour happened because the shouldShowBackdrop
constant had a false value which was being taken into consideration when
computing the backdrop opacity for the ionic theme. This PR fixes this
by always transitioning the backdrop opacity from 0 to 0.7 once the
modal appears, independently of the initial, current or activation
breakpoint.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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