fix name in comment header
android: use the 'am' utility to start the pinetry GUI
android: use a file-backed socket

Using a file backed socket (as opposed to one in the abstract namespace) lets us easily control the access permissions. I also significantly cleaned up the code to make it more readable.
android: pass our linux uid to the GUI Activity

so it knows which socket to connect to
android: handle internal and external pinentry processes correctly
android: enforce owner permissions on the socket
android: use SO_PEERCRED to authenticate the connected client

In all cases we only want the actual gnupg-for-android application
to connect to our pinentry socket. If this is an internal pinentry,
the uid of gnupg-for-android is our uid. If it is an external pinentry,
we can determine the uid of gnupg app with a high level of confidence
by checking the owner of the "/data/data/info.guardianproject.gpg".
android: use abstract domain socket for external invocations
android: disable debug messages
android: remove hardcoded GNUPGHOME and use environment variable instead
android: remove ifdefed code that launched the GUI via another socket
android: fix warnings and remove unused function