Move SDK prebuilts to new prebuilt_sdk module type

SDK prebuilts need to be treated differently than other prebuilts.
They are used like a normal Java prebuilts when compiling java files,
but they are also used as aapt includes when compiling apps, and they
provide prebuilt aidl files when compiling aidl sources.  Create a
new module type so dependers can determine what kind of dependency
they are.

Change-Id: If426da63cfadcb4b14abda58c7b346554912a07a
diff --git a/cmd/soong_build/main.go b/cmd/soong_build/main.go
index 83205ba..d17fbed 100644
--- a/cmd/soong_build/main.go
+++ b/cmd/soong_build/main.go
@@ -67,6 +67,7 @@
 	ctx.RegisterModuleType("java_binary", java.JavaBinaryFactory)
 	ctx.RegisterModuleType("java_binary_host", java.JavaBinaryHostFactory)
 	ctx.RegisterModuleType("prebuilt_java_library", java.JavaPrebuiltFactory)
+	ctx.RegisterModuleType("prebuilt_sdk", java.SdkPrebuiltFactory)
 
 	// Mutators
 	ctx.RegisterEarlyMutator("arch", common.ArchMutator)