Move zygote startup logic to the frameworks.

The Zygote class is now in com.android.internal.os. It is
responsible for the vast majority of work before and after
the call to fork(). It calls back into the Runtime via
the new dalvik.system.ZygoteHooks class to allow the Runtime
to perform pre fork cleanup and post fork initialization.

The native code in Zygote.cpp is a direct and straightforward
port of the existing code in art. Most differences are
superficial, for example :
- We use C style logging (ALOGE) instead of stream based
  logging.
- We call env->FatalError() instead of using LOG(FATAL)

Change-Id: Ia101fb2af12d23894fe57e4134d2bc6d142e5059