Switch aidl::nullable to std::optional

AIDL's @nullable is represented as std::optional in C++ backend.
The previous aidl::nullable was a temporary solution to avoid
merge-conflicts between rvc-dev and master branches.

Bug: 151275535
Test: m
Change-Id: I8fe5bfc1763087e7ff364b4408f862436492d1db
3 files changed