[Android/IME] Add tests for the current PoC

[Reinmar]

In #12058 we created a PoC without tests so far.

The PoC turned out to work really well, so in order to stabilize and merge it to the main IME branch, we need to add tests coverage.

[Reinmar]

One thing that we found while pre-reviewing the PoC in #12446 is what are those changes:

https://github.com/ckeditor/ckeditor5/pull/12446/files#diff-8e0d7c69269756056f12fa2ffff70ee41cd4037b6725fd1e4af279c84ac8686dR41

They seem to be only partially related to Android, but were added in this PoC. We need to at least understand why they were made.

[Reinmar]

IMPORTANT: Let's not try to fix new cases that we'll discover when working on this PR. Extract them and target separately.

[niegowski]

Closing as part of https://github.com/ckeditor/ckeditor5/issues/12058.