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: I90cf2ce1193c479179687d71a5c1416f6cdf0b16
Change-Id: I90cf2ce1193c479179687d71a5c1416f6cdf0b16
(cherry picked from commit ff7f4279bd7fa33577f5f4d46aa9de322fb625d4)

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