update: reject APKs with invalid file sig, probably Janus exploits

This just checks the first four bytes of the APK file, aka the "file
signature", to make sure it is the ZIP signature and not the DEX signature.
This was checked against the test APK, and I ran it against some known
malware and all of f-droid.org to make sure it works.

All valid ZIP files (therefore APK files) should start with the ZIP
Local File Header of four bytes.

https://www.guardsquare.com/en/blog/new-android-vulnerability-allows-attackers-modify-apps-without-affecting-their-signatures