Switch to $(file) instead of a rule to create soong.variables

Kati seems to have some problems updating this file, and the echo
syntax gets a bit messy. So define some macros and write out the file at
eval time using $(file).

Test: multiproduct_kati -only-config, check soong.variables (on AOSP and internal)
Change-Id: I74279b962918ca1f70fb1d0d25cbb30bd5675041