When using SEND intent with SUBJECT empty, make TEXT the first field

AnkiDroid accepts the generic Android SEND intent, but it expects the Front to be in EXTRA_SUBJECT and the back to be in EXTRA_TEXT. As per [forum thread](https://groups.google.com/forum/#!topic/anki-android/8Tu5tUlZgmI), generic apps like Gmail can send a selected word via SEND, but it sends a single word in EXTRA_TEXT. 

In hindsight, having EXTRA_SUBJECT map to the first field was not an optimal design decision. It makes much more sense to have SUBJECT map to the second field, but we can't really change it now, so I've implemented this workaround.