multimodule support: move strings to `:common:android`

> **Strings**: move all into `:common:android`.
>
> This unblocks us simply/quickly and allows a future discussion about the CrowdIn sync/file architecture.
>
> ----
>
> Medium-term: I think `:widgets` containing strings will be a "Won't fix" for a while
>
> Architecturally, strings should live in their modules, but I think moving strings up into `:widgets` would:
> * Mildly improve `R.string` via namespacing
> * Produce minimal performance gains (the resources would still need to be bundled inside the app).
> * Not improve discoverability (developers know where the strings are)
> * Add disruption/infra complexity in the case that a string needs to be pulled up/down from `:widgets` into `common:android`
> * Also with lint & cross-module duplication checks
> * Not have any tangible benefit to translators
>
> I don't see the upsides being worth the time to update our translation infra for this.

_Originally posted by @david-allison in [#20737](https://github.com/ankidroid/Anki-Android/issues/20737#issuecomment-4303120972)_