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

Exempt-From-Owner-Approval: cherry-pick from internal

Merged-In: Id0fead2717b1f6a3b9a1867efa8b588b59d3fbd2
(cherry picked from commit 3b25ae15)
Change-Id: Id0fead2717b1f6a3b9a1867efa8b588b59d3fbd2