Remove support for --classpath and make --runtime-init the default.

--classpath hasn't been properly supported or used for as long as
git history exists, and the non --runtime-init classpath is useless
without it.

Also, forking and exec' a bare dalvikvm instance from the zygote seems
somewhat pointless since we might as well just execute it directly.

Change-Id: Ib07d822d448e7b431d7564381f9cd11735fd1113