Your problem is simple:

your android device is compromised, somebody hacked it and got your private keys. Most probably, the person hacked it days before.
Then, the hacker went to blockchain.info, introduced your keys and decided to robe only half. Why only half? Because probably the hacker knows you and didn't want to robe all your bitcoins, for emotional reasons.

So, who has been using your phone lately?
How many intelligent related people has access to your things? A son, a brother, uncle, a coworker, friend.... the fact that s/he returned half must tell you something.

Pin protecting the screen is useless. That protection can be very easily eliminated. The attacker could even have copied  your phone flash image into a computer in 2 minutes and return it to you without you noticing anything. Most phones are hackeable very easy just by knowing how to flash a ROM. Does any of your related know how to do so? Did you left your phone in some place for a while?

Note that the hack could have been happened in any time since you installed the application. Can be 1 week or 1 year or any time. So try to remember when you exposed your phone in such a way.