Mark BUILD_NUMBER as obsolete, more vars as readonly

In preparation for moving BUILD_NUMBER and FILE_NAME_TAG handling to
soong_ui, mark BUILD_NUMBER as obsolete and FILE_NAME_TAG as readonly.

Later changes will remove BUILD_NUMBER entirely, and move FILE_NAME_TAG
handling into the kati packaging step via dist-for-goals renames.

Test: build_test on master
Change-Id: I0e340d32fe0cb6296d3b5a7fec349b50632d044e