scan APKs for signs of "Master Key" exploit

scan APKs for signs of "Master Key" exploit

This exploit is old, and was fixed in 4.4.  But it was easy to exploit,
so it is still worth scanning for it.  It is also easy to scan for, since
valid APKs should not have files with duplicate names.  In theory, this
could look for duplicate file names for any file, but this limits the
false positives by only checking names of files related to executing code.

fdroidclient#40