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: I6bf4c2017f113f4d326fddb1d76163c2fed34d50
Change-Id: I6bf4c2017f113f4d326fddb1d76163c2fed34d50
(cherry picked from commit 0e88d61d)

Exempt-From-Owner-Approval: CP from master