onAppPrivateCommand property Null safety

This is used to receive a private command from the input method.

Called when the result of TextInputClient.performPrivateCommand is received.

This can be used to provide domain-specific features that are only known between certain input methods and their clients.

See also:

• https://developer.android.com/reference/android/view/inputmethod/InputConnection#performPrivateCommand(java.lang.String,%20android.os.Bundle), which is the Android documentation for performPrivateCommand, used to send a command from the input method.
• https://developer.android.com/reference/android/view/inputmethod/InputMethodManager#sendAppPrivateCommand, which is the Android documentation for sendAppPrivateCommand, used to send a command to the input method.