Add policy for apexd.

apexd is a new daemon for managing APEX packages installed
on the device. It hosts a single binder service, "apexservice".

Bug: 112455435
Test: builds, binder service can be registered,
      apexes can be accessed, verified and mounted
Change-Id: I634ad100f10b2edcd9a9c0df0d33896fa5d4ed97
18 files changed