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: I16c3e85c195b878d3c629149afbd96636ed1f0e1