binder: ndk: add asBinderReference method to BpCInterface

This is more efficient than `asBinder` when the caller doesn't need to
increment the ref count. Used in https://r.android.com/3211938

Test: m
Bug: 338458975
Change-Id: I15dd9b5c1033c58760e1038f09190bbbafc4bf49

Author: fkm3

libs/binder/ndk/include_cpp/android/binder_interface_utils.h

@@ -225,6 +225,8 @@ class BpCInterface : public INTERFACE {
 
     SpAIBinder asBinder() override final;
 
+    const SpAIBinder& asBinderReference() { return mBinder; }
+
     bool isRemote() override final { return AIBinder_isRemote(mBinder.get()); }
 
     binder_status_t dump(int fd, const char** args, uint32_t numArgs) override {