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: Ie3196ee5cce17d77950eea9479d2cc1406e9e674
Merged-In: I33822bc76ef87637d5408849f64a0607e121792e
Change-Id: I33822bc76ef87637d5408849f64a0607e121792e
(cherry picked from commit ad62e8cb)

Exempt-From-Owner-Approval: cherry-pick from master with owner's approval