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: I2b87761401361f9cf96cdda070f26e70a5c6c935
(cherry picked from commit b8ba2341)
Change-Id: I2b87761401361f9cf96cdda070f26e70a5c6c935