Better Soong config namespace support.

Old macros (`add_soong_config/namespace` & `add_soong_config_var_value`) are
going away, to be replaced with `soong_config_set`. A new macro,
`soong_config_append` is added to append to the existing value.
Also, flag the attempts to reference the values of the variables in the
Soong config namespace.

Bug: 200297238
Test: internal
Change-Id: Idb6a31632db75d7faef038b83e6a86d9dcf1e736