Improve build.xml: git, version strings, apk names

Do not use local ./git script, it was a hack for Eclipse that was
platform-specific and worse, failed with infinite recursion if $PATH
contained the "." directory.

Use the improved Serval DNA version_string.sh script to compute the
version string.

Produce APK files named -alpha, -beta, -release instead of naming
them all -release.

Replace "upload" target with "upload-alpha", "upload-beta" and
"upload-release".

Safety check: ensure that the configured SDK and NDK directories
actually look like an SDK and an NDK.