Add Asuite prebuilds in PATH and remove functions.

Remove atest/acloud/aidegen functions and append prebuilt path
in PATH. Use local built by default; if there is no local built,
use the prebuilt one instead.

Bug:

Test: $ source build/envsetup.sh; lunch
      $ echo $PATH|sed 's/:/\n/g' | egrep "out|asuite"
      # will see out/host prioritises prebuilts/asuite.

Change-Id: I1e5869bcb81cf7058cf8971f480a47d2c8f8b6f0