Add aidl::nullable type for nullable in AIDL

This provides source-level compatibility between new(std::optional) and
old(std::unique_ptr).

Bug: 144773267
Test: m
(cherry picked from commit debae060)

Change-Id: I258f3bf66b168c4ffe3e528ac1dc5be2076a3b66
Merged-In: I378dbf848a51a705a9e06f9cd4b0def9728c3bff
Exempt-From-Owner-Approval: approved in master.