unique_fd is passed by value in AIDL interfaces

FileDescriptor type in AIDL was translated into const unique_fd& in C++.
Now, it is unique_fd, i.e. passed by value, to make it easier to keep it
beyond the scope of the call.

Bug: 144943748
Test: m
Change-Id: Ic6b1a4cba71c0fedb206b5ca3fb65b9944bbd69f
3 files changed