Add skeleton of test helper framework

This adds two directories under cts for the device interaction helper
framework:

* libs/helpers will contain support classes for building helpers and
  defining their interfaces.
  * libs/helpers/core is for support classes and the top-level
    ICtsTestHelper interface.
  * libs/helpers/interfaces is for per-module helper interfaces.
* helpers will contain concrete helper implementations.
  * helpers/default is for Google-provided helpers that drive the
    default Android/AOSP UI.
  * helpers/vendor will be for OEM-supplied/owned implementations to
    drive their device-specific customizations.

Bug: 132747867
Test: m cts_v2
Change-Id: If43263ba933bc53814f173852ebd868d997713fa
Merged-In: If43263ba933bc53814f173852ebd868d997713fa
(cherry picked from commit ba96060bf6a32c6e82d98ffb486e2a1ec67d07d8)