Introduce a new tool, aconfig, to manage build time configurations, such
as feature flags.
This CL adds a project skeleton. Later CLs will add the actual
implementation.
Note: there is no need for an explicit TEST_MAPPING file;
rust_test_host are automatically included in host-unit-tests and
explicitly adding these tests via TEST_MAPPING will cause an error.
Bug: 279485059
Test: m aconfig && aconfig
Test: atest aconfig.test
Change-Id: I94047a19a55ae4d45b4fd7a0c8105a91ddcdfd79
