Request Incognito mode on keyboard

It would be nice if Element could request keyboard incognito mode (on a per-chat-basis). While incognito mode is active, the keyboard should stop "learning" the words you are typing. Several keyboards have it implemented.
Element could use this flag: [IME_FLAG_NO_PERSONALIZED_LEARNING](https://developer.android.com/reference/android/support/v13/view/inputmethod/EditorInfoCompat#IME_FLAG_NO_PERSONALIZED_LEARNING).