fix(android): Catch when WebView provider unavailable

[darcywong00]

Fixes: #11560
Fixes: KEYMAN-ANDROID-5E9
Fixes: KEYMAN-ANDROID-6MA

Keyman requires system WebView to be installed and enabled.

This updates KMManager to catch when the system WebView is not installed/enabled, and attempt to notify via Toast.

At the moment, the Toast notification doesn't appear when the system keyboard fails to initialize. I suspect there's no IME available to display the message...

User Testing

Setup - Install the PR build of Keyman for Android on an Android device/emulator.
Launch Keyman and enable as the default system keyboard. Then close the app.
Next, go to system Settings --> Apps --> expand and find "Android System WebView --> Disable.
This disables the system WebView which Keyman requires to run.

• TEST_INAPP_KEYBOARD - Verifies inapp keyboard doesn't crash when WebView disabled

1. Launch Keyman for Android

2. Observe the splash screen and verify a Toast notification appears about the WebView failing
   

3. Verify Keyman automatically closes without errors about crashing

• TEST_SYSTEM_KEYBOARD - Verifies system keyboard doesn't crash when WebView disabled

1. Launch the Chrome browser and select a text field
2. Verify Keyman system keyboard fails to appear.
3. Verify device doesn't show errors about Keyman crashing.

[keymanapp-test-bot]

User Test Results

Test specification and instructions

• ⬜ TEST_INAPP_KEYBOARD (OPEN)
• ⬜ TEST_SYSTEM_KEYBOARD (OPEN)

Results Template

# Test Results

* **TEST_INAPP_KEYBOARD (OPEN):** notes
* **TEST_SYSTEM_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]

From Slack, @mcdurdin suggests replacing Toast with a label (more persistent)

Hmm, I think the system keyboard should have a button to 'switch to another keyboard app' (if one is available), but the in-app should just show a message that they need to enable webview.
if it is possible to launch the settings activity where the webview setting is, that'd be great