Get build-related vars only one time

This change introduces a class that stores build-related variables, and
functions `get_build_top`, `get_product_out` and `get_host_out` that are
able to translate multiple strings to Path.

Bug: 249956928
Test: atest-dev atest_unittests --host
Change-Id: I0f23ae641b15a66260e8a0bbbe365d7df8575c86
3 files changed