Add support for writing all Soong config variables into @soong_injection.

Also remove the need to use bp2build_available on
soong_config_module_types as we want to convert every single of them
into the tree for a complete soong_injection soong_config_variables.bzl
file.

The variables are split into their bool, value and string types
respectively, as they all need to be handled differently on the Bazel
product_platform side, as well as for generating constraint values and
settings. For example, value variables need to integrate with
TemplateVariableInfo, and string variables need to include the string
value itself into the select key/constraint value.

Sample soong_config_variables.bzl file: https://gist.github.com/jin/cef700bfb20c8656a931306dd71d47e1

Test: CI
Bug: 198556411
Change-Id: I8665dd1269a507edb37de62407ed3641564bea5c