Use optional for nullable types

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: I98549c8614c9152d5d45e2f1f33f2f3c31a9bbbf
Change-Id: I98549c8614c9152d5d45e2f1f33f2f3c31a9bbbf
(cherry picked from commit 3ce0ee5363c9325e5c95dadbe24a03d94aedc90e)

Exempt-From-Owner-Approval: CP from master
2 files changed