Teach aconfig about build IDs (continuously increasing integers). Extend the aconfig file format to allow flags to say "by default, my value is X, but starting from build ID A, it's Y, and from build ID B, it's Z". Bug: 279485059 Test: atest aconfig.test Change-Id: Idde03dee06f6cb9041c0dd4ca917c8b2f2faafdd