AIDL generates optional<T> for nullable T types for C++, which is more
efficient and idomatic and easy to use.
Bug: 144773267
Test: build/flash/boot
Merged-In: I90cf2ce1193c479179687d71a5c1416f6cdf0b16
Change-Id: I90cf2ce1193c479179687d71a5c1416f6cdf0b16
(cherry picked from commit 53be7f4db497a05256407bd4c499a8fafc8e095d)
Exempt-From-Owner-Approval: CP from master