Add support for predictive back animations in Firefox for Android 15

[Jaime]

Since the launch of Android 15, many smartphones have started receiving this OS version and will continue to do so in the coming months.

Android 15 introduces predictive back animations, which can be opted into by each app.

This suggestion proposes integrating them into the Firefox app for Android to align it more closely with the native behavior of system apps.

More information: https://developer.android.com/guide/navigation/custom-back/support-animations

[Jon]

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