Revert "Revert "Add aidl::nullable type for nullable in AIDL" am..."

Revert submission 10645536-am-7de918011fdf43d4b69d7fb2a8dc33d6

Reason for revert: Unintended revert got merged.
Reverted Changes:
I1b0cd0bc7:Revert "Add aidl::nullable type for nullable in AI...
I82f97aa26:Merge "Revert "Use aidl::nullable for nullable typ...

Change-Id: I59d2d6b6ebacbc598b63788e4a5c8ad6e005ccaa

Exempt-From-Owner-Approval: reverting unintended revert