Move Fullscreen enum into platforms

[madsmtm]

This is a bit of duplication, but needed if we intend to split the backends into separate crates, since we need to convert between the root crate's VideoModeHandle and MonitorHandles, and the platform's types.

[MarijnS95]

How does this work if the functions implementing the API for this (that take or return the Fullscreen type) are defined by a core trait?

In any case, on Android we can remove this entirely because it's "not implemented" and not entirely sane to implement.

[kchibisov]

Given that the general movement recently to completely remove platform_impl::Type things, I'm not sure what to do about this one...

[kchibisov]

since we need to convert between the root crate's VideoModeHandle and MonitorHandles, and the platform's types.

We don't actually want to convert though, we want to use the exact same thing on the top type as well, like for Window right now, we don't have any conversion.