Move multidex support library

Multidex is being moved from frameworks/support/multidex to
frameworks/multidex.

GMS Core will be using the multidex library and has gotten special
approval to use the unreleased multidex library (i.e., guinea pig).

Bug: 10674263
Change-Id: I31f582dd7adf4db6eb19c52357875594fc9f7829