Do not override setting of SOONG_CONFIG_<m>_module_source_build

Previously, any setting of SOONG_CONFIG_<m>_module_source_build (where
<m> is one of wifi, uwb or bluetooth) that was made before the
core/android_soong_config_vars.mk was processed would be replaced with
false. This change will only set them to false if they have not been
previously set.

Bug: 233965247
Test: m nothing
      # After setting SOONG_CONFIG_wifi_module_source=true before this
      # code. Without this change out/soong/soong.variables uses false
      # for the wifi_module/module_source config variable. With this
      # change it uses true.
Change-Id: I1dffeb055a68250f574630d4ef5fedc51bad4a2b