for ant build, run clock at 5% speed

This should help produce reproducible timestamps in the APK file.
remove .sh from script so shebang can be easily changed

These scripts need not be implemented in /bin/sh, that's just an
implementation detail that should be hidden.
remove ant.properties from git, it is not used at all

The make-release-build script also will copy ant.properties into place to
setup the release signing key.
only gpg-sign a release build if gpg is available with secret keys

This makes it easy for people to run this script to verify the official
release build matches their build.