Read product variables from soong.variables

Refactor the soong.config loading code to support reading in
product variables from soong.variables.

Change-Id: I389e6bb5c501b53167267d5f5d0d25557811cf72
diff --git a/androidbp/cmd/soong.go b/androidbp/cmd/soong.go
index e9d421a..65a4239 100644
--- a/androidbp/cmd/soong.go
+++ b/androidbp/cmd/soong.go
@@ -173,7 +173,7 @@
 	"BUILD_JAVA_LIBRARY":   "BUILD_HOST_JAVA_LIBRARY",
 }
 
-var productVariableConditionals = map[string]struct{conditional, value string}{
+var productVariableConditionals = map[string]struct{ conditional, value string }{
 	"device_uses_jemalloc": {"ifneq ($(MALLOC_IMPL),dlmalloc)", ""},
 	"device_uses_dlmalloc": {"ifeq ($(MALLOC_IMPL),dlmalloc)", ""},
 	"device_uses_logd":     {"ifneq ($(TARGET_USES_LOGD),false)", ""},