Make Magikeyboard work under Samsung Dex (like it was done for KeePassDX in #1114)

**Is your feature request related to a problem? Please describe.**
Though KeePassDX app itself was "enabled" to work with Samsung Dex (fix/merge #1114 addressed #245 #537 #963), it looks like Magikeyboard is not yet fully supported/updated to work under Dex
Sympthoms:
1. Magikeyboard is not listed in phone's settings where one enables/disables installed keyboards.
2. Magikeyboard is not listed on "Choose input method" Android pop-up when to press "keyboard selection" key of gboard/samsung keyboard
3. "Activate a custom keyboard ..." info is displayed under "Form filling" setting of KeePassDX app
4. If to exit Dex mode, Magikeyboard becomes visible in KB selection and works as expected.

**Describe the solution you'd like**
Update Magikeyboard to work under Samsung Dex

**Describe alternatives you've considered**
Obvious one is not to use Magikeyboard and just use copy-paste via clipboards, but it's less safe than via MK and obviously less user-friendly.

**Additional context**
Screenshots - attached (one with MK visible was done in non-dex mode).
I've tried to change where the keyboard is displayed in Dex mode (either on external monitor/display or on the phone), but it doesn't matter - MK is not listed.


Start by reviewing the Magikeyboard implementation and comparing the KeePassDX changes referenced in #1114, then reproduce the issue in Samsung DeX. Done means Magikeyboard appears in Android keyboard settings and input-method selection while DeX is active, and the custom-keyboard warning is resolved.

Written by the indexing model from the issue text.