Fix Share Intent behaviour 

This commit fixes odd textsecure behavior with share intents:
* when share intents with mime type "\*/\*" were handled (containing multiple types, for example a text and an image), the contents were completely ignored and the user saw an empty text field.
* share intents with typ "image/*" (or other media types) would attach the image to the draft correctly, but ignored any text that was possibly included. 


This commit changes the behaviour in the following ways:
* the EXTRA_TEXT string from the intent is always included
* the mime-type of the Uri passed in EXTRA_STREAM is deduced via ContentResolver or File extension, depending on the Uri type which makes handling of "\*/\*"-type intents possible (and possibly makes textsecure more robust against wrong intent types)

This behaviour is the same as in most other apps (tested Hangouts, several mail apps, G+ and others)