Android: clear the native focus when the keyboard hides

Otherwise the keyboard will popup again when the app is switched to the
background and back.

Fixes #5235

internal/backends/android-activity/java/SlintAndroidJavaHelper.java

@@ -409,6 +409,7 @@ public class SlintAndroidJavaHelper {
             public void run() {
                 InputMethodManager imm = (InputMethodManager) mActivity.getSystemService(Context.INPUT_METHOD_SERVICE);
                 imm.hideSoftInputFromWindow(mInputView.getWindowToken(), 0);
+                mInputView.clearFocus();
                 mInputView.setCursorPos(0, 0, 0, 0, 0, 0);
             }
         });