Add CheckValid to AidlMethod

The content of AidlInterface::CheckValid can be split into validation
for the interface and validation for each method to ease readability.

Test: aidl_unittests
Change-Id: I3c14e1d28b00e50447190e7cc24df4b19ccc2110
diff --git a/aidl_language.h b/aidl_language.h
index 3118b7c..b6cc448 100644
--- a/aidl_language.h
+++ b/aidl_language.h
@@ -829,6 +829,7 @@
   AidlMethod& operator=(const AidlMethod&) = delete;
   AidlMethod& operator=(AidlMethod&&) = delete;
 
+  bool CheckValid(const AidlTypenames&) const;
   const AidlTypeSpecifier& GetType() const { return *type_; }
   AidlTypeSpecifier* GetMutableType() { return type_.get(); }