Support select in combined_apis properties

This change allows combined_apis.bootclasspath and
combined_apis.system_server_classpath properties to be modified based on
configuration using select statements.

Bug: 349800749
Test: m nothing --no-skip-soong-tests
Change-Id: I08bf4200738268fe833389f15e24271058528944
diff --git a/api/Android.bp b/api/Android.bp
index d4ba964..4de3a28 100644
--- a/api/Android.bp
+++ b/api/Android.bp
@@ -29,12 +29,14 @@
     pkgPath: "android/soong/api",
     deps: [
         "blueprint",
+        "blueprint-proptools",
         "soong",
         "soong-android",
         "soong-genrule",
         "soong-java",
     ],
     srcs: ["api.go"],
+    testSrcs: ["api_test.go"],
     pluginFor: ["soong_build"],
 }