[JetNews] Fix state saving with drawer

[alexvanyo]

Fixes the state saving (via rememberSaveable) throughout the app.

Per the design, we conditionally want to allow the drawer to be shown based on the size of the screen. With the current approach we conditionally passed null or the drawer content to a top-level Scaffold, which would then allow the drawer to be hidden or shown. If we don't want the drawer to be shown, we instead show the nav rail.

This approach has a subtle problem due to the implementation of Scaffold: when we switch the drawer content between null and non-null, we are changing the location of where the content lambda (containing everything else in the app) is called.

This changes all of the keys that rememberSaveable automatically uses, which is seen as scroll positions not being saved when switching between orientations that allow or disallow showing the drawer.

To workaround this, I replaced the top-level Scaffold with a ModalDrawer (since that was the only functionality we were using via the top-level Scaffold directly anyway). Crucially, this ModalDrawer is always called, even if we don't want to allow showing the drawer. This allows calling the content for the rest of the app in exactly one source location. To avoid showing the drawer when we want to show the nav rail instead, we switch between a real, remembered DrawerState and a fixed-to-closed drawer state.

[manuelvicnt]

We should move these variables closer where they're used. Shouldn't be above BWC

[alexvanyo]

Updated as part of the extraction into rememberSizeAwareDrawerState

[alexvanyo]

Thanks @manuelvicnt !