aconfig: add support for changing flag value based on build

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