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

Changes.md

@@ -1,5 +1,22 @@
 # Build System Changes for Android.mk Writers
 
+## `BUILD_NUMBER` removal from Android.mk  {#BUILD_NUMBER}
+
+`BUILD_NUMBER` should not be used directly in Android.mk files, as it would
+trigger them to be re-read every time the `BUILD_NUMBER` changes (which it does
+on every build server build). If possible, just remove the use so that your
+builds are more reproducible. If you do need it, use `BUILD_NUMBER_FROM_FILE`:
+
+``` make
+$(LOCAL_BUILT_MODULE):
+	mytool --build_number $(BUILD_NUMBER_FROM_FILE) -o $@
+```
+
+That will expand out to a subshell that will read the current `BUILD_NUMBER`
+whenever it's run. It will not re-run your command if the build number has
+changed, so incremental builds will have the build number from the last time
+the particular output was rebuilt.
+
 ## `DIST_DIR`, `dist_goal`, and `dist-for-goals`  {#dist}
 
 `DIST_DIR` and `dist_goal` are no longer available when reading Android.mk