imetests: Add a regression test for Polish keyboard input

Using AltGr+a to input 'ą', then inputting 'a'.

I reverse-engineered how to do this based on David's older commits.

Bug: T69367
Change-Id: I94e52fc437af729ff8cfa814346d8c338233c612
ve.ce: Remove hacks from Surface#onDocumentKeyPress; add related IME test

We had a hack in place to filter out non-character keys, to prevent
unexpected content deletion (caused by pawning) when selection is not
collapsed and the user presses, for example, the Home key (Firefox
fires 'keypress' for it). Pawning is gone since 40101e42 and this
seems no longer needed.

Added an IME test for the described situation.

Bug: T69367
Change-Id: I277b92b9d11bfda01a92f3e615ca674feb920a1a