libbinder_ndk: backwards compatible include_cpp

These internal C++ binder APIs are shipped with the SDK out of
convenience, but they still need to work when used with old versions of
the AIDL compiler. Until we have a better solution, making them
backwards compatible.

Bug: 160624671
Test: build success, but when either of the two branches is hit, it's
  the expected build failure.
Change-Id: I62d68696d86e43c35f2006f59a007299eb096d91