[remake] Restore old back handling for FlutterFragmentActivity (#161545)

Remake of https://github.com/flutter/engine/pull/56565. Quoting from an
old comment, slightly edited:

------------ 

https://github.com/flutter/engine/pull/52302 seems to have
unintentionally had the effect of not allowing people to "opt out" of
predictive back. This is actually aligned with what the android docs say
should happen:

https://developer.android.com/guide/navigation/custom-back/predictive-back-gesture
> Note: OnBackPressedCallback is always called regardless of the value
of android:enableOnBackInvokedCallback. In other words, disabling the
system animation doesn't affect your app's back handling logic if it
uses OnBackPressedCallback.

But this wasn't actually true for flutter apps before
https://github.com/flutter/engine/pull/52302, because we were not
calling `super`, and `FlutterFragmentActivity` extends a
`FragmentActivity` which in turn extends a `ComponentActivity`, which
uses the old `onBackPressed` to [invoke the new back
handling](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:activity/activity/src/main/java/androidx/activity/ComponentActivity.kt;l=587?q=ComponentActivity):
```kotlin
   override fun onBackPressed() {
        onBackPressedDispatcher.onBackPressed()
    }
```

So while the docs imply that removing the `onBackPressed` in
`FlutterFragmentActivity` shouldn't have had an effect, that wasn't true
because in our case we were consuming the back event and ignoring the
warning
```java
  @Override
  @SuppressWarnings("MissingSuperCall")
  public void onBackPressed() {
    flutterFragment.onBackPressed();
  }
```

What all this means is that apps that _aren't_ opting in to predictive
back had their back handling migrated to the new code path
automatically. FlutterFragmentActivity was uniquely is forced into the
new back handling codepath by
https://github.com/flutter/engine/pull/52302, which this PR fixes.

------------ 

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- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

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---------

Co-authored-by: Gray Mackall <mackall@google.com>