Introduce --multi-dex option in dx.

Introduce --multi-dex option in dx.

Add --multi-dex options to dx command line to allow the generation of
several dex files when method index limit is about to be reached.
Also add
  --main-dex-list allowing to force some classes in the main dex.
  --minimal-main-dex to keep in main edx only classes specified by
      main-dex-list.
  --set-max-idx-number to set an arbitrary idx limit for the
      splitting.

(cherry picked from commit c7daf656da3a4854296b6a8bb702e3ee418450e5)

Change-Id: I2b42272be91484a75783eb94cd30581159948975