Use aidl::nullable for nullable type in C++

To reduce the discrepance between old code which still uses
std::unique_ptr and new code using std::optional.

This might help to avoid merge-conflicts between branches.

Bug: 144773267
Test: m

Merged-In: I33822bc76ef87637d5408849f64a0607e121792e
Change-Id: I08b23d5bc08655b89ea89a06dad4830d14b070bb
Exempt-From-Owner-Approval: approved from master
(cherry picked from commit c85cc13b)