fix(android): use main looper to dispatch key events when OSK is hidden

[jahorton]

Fixes: #12366
Supersedes: #12381

Alas, it would appear that we cannot rely on the WebView's version of postDelayed, as apparently that doesn't actually trigger delayed events when the WebView has been detached! We can, however, construct a Handler instance and link it to the UI thread's looper in order to achieve our desired effect.

User Testing

Setup - Install the PR build of Keyman for Android on a physical device. Also pair an external keyboard (USB/bluetooth) to the device

TEST_KEYBOARD - Verifies external keyboard works when virtual keyboard hidden

1. Launch Keyman
2. From "Get Started", enable Keyman as the default system keyboard
3. Launch a separate app where you can type in (ideally a note taking app like Keep or OneNote as reported in the issue). If those aren't available, Chrome browser works
4. Select a text area with Keyman sil_euro_latin selected as the system keyboard
5. Verify the system keyboard types fine (longpresses, switching, keys, etc).
6. With the virtual keyboard displayed, type on the physical keyboard ` a
7. Verify the text becomes à.
8. From the Android system preferences, select "Language and Keyboard" --> Physical keyboard --> Toggle "show on-screen keyboard" so that the OSK is hidden
9. Return to the note-taking / Chrome app and select a text field
10. Type on the physical keyboard ` a
11. Verify the text becomes à

Please repeat the test a few times, verifying that no errors are triggered during any attempt.

[keymanapp-test-bot]

User Test Results

Test specification and instructions

• ⬜ TEST_KEYBOARD (OPEN)

Results Template

# Test Results

* **TEST_KEYBOARD (OPEN):** notes

Test Artifacts

• Android
  • Keyman for Android apk
  • FirstVoices for Android 16.0 apk
  • FirstVoices Keyboards for Android apk
  • KeyboardHarness apk
  • Keyman for Android 16.0 apk
  • KMSample1 apk
  • KMSample2 apk

[darcywong00]

lgtm.
Thanks for finding this!

[darcywong00]

Can you also 🍒 pick to 17.0 so we can unblock the community site user?

nvm - I made the 🍒 pick in #12875