Build only 64-bit odex for system server jars.

By default we build both 64-bit and 32-bit odex files for a Java library.
With this change:
- For system server jars (PRODUCT_SYSTEM_SERVER_JARS), we build only
  64-bit odex;
- A library can opt to build only 64-bit odex with
  "LOCAL_MULTILIB := first".

Bug: 19650934
Change-Id: Ic0b7fd381396ed276e6129f883881c5c41c6e154