Add conditions_default for soong config variables.

Each variable can specify a conditions_default for properties to be used
when the variable is not set, not set to a true value (for bools), or is
set to a value that is not present in the module (for strings).

Test: m nothing
Test: go test soong tests
Change-Id: I76ec026da2369b407f0f530f77760f530e7958fc
diff --git a/android/soongconfig/Android.bp b/android/soongconfig/Android.bp
index df912e6..6bb68eb 100644
--- a/android/soongconfig/Android.bp
+++ b/android/soongconfig/Android.bp
@@ -10,4 +10,7 @@
         "config.go",
         "modules.go",
     ],
+    testSrcs: [
+        "modules_test.go",
+    ],
 }