Provide better abstraction of class Slang.

1. Remove the Slang C APIs support (not used by any other programs.)
2. All RS relatives are now in slang_rs_*.cpp.
diff --git a/Android.mk b/Android.mk
index cb73452..fd7b60f 100644
--- a/Android.mk
+++ b/Android.mk
@@ -61,11 +61,11 @@
 
 LOCAL_SRC_FILES :=	\
 	slang_driver.cpp	\
-	libslang.cpp	\
 	slang.cpp	\
 	slang_backend.cpp	\
 	slang_pragma_recorder.cpp	\
 	slang_diagnostic_buffer.cpp	\
+	slang_rs.cpp	\
 	slang_rs_context.cpp	\
 	slang_rs_pragma_handler.cpp	\
 	slang_rs_backend.cpp	\