Clean up argc / argv processing for runtime args.

- Make copies of argc, argv before argv is potentially
  overwritten with the process name.
- Allow multiple command line arguments to be passed to
  ZygoteInit (this is required for some of the 64 bit
  zygote work).
- Add an explanatory comment about how these argments
  are processed.

Change-Id: I752be69c5c0f97ed17d1a3dded19f46ee00929b0