Android: Refactor Custom Build options in export preset

Instead of reusing the custom_template/ prefix which is actually only
used for the prebuilt APK workflow, we add a new custom_build/ prefix.

This is a slight compat breakage (users will have to redo their config)
but enables us to group the Min SDK and Target SDK options where they make
sense, and avoid reusing the previously hardcoded Target SDK 30 from
Godot 3.4.

Those two options are now strings instead of integers so that we can keep
them empty by default, and show their default value using a placeholder.
So some validation has been added to make sure they are proper ints.
The upper bound on Target SDK was also removed as it's a common use case
to use it to try to target newer released SDKs. But we warn the user that
this wasn't validated by us.

The export info dialog is now exclusive so that when it doesn't auto-close,
i.e. when it errors, you don't close it by mistake by clicking outside.

Fixes #62465.

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