ParcelFileDescriptor is movable

Add a move constructor to ParcelFileDescriptor. This is required to be
able to something like

vector<ParcelFileDescriptor> fds;
fds.push_back(ParcelFileDescriptor(std::move(unique_fd(open("some_file")))));

Bug: 115607973
Test: system/tools/aidl/runtests.sh
Change-Id: If8186500b528984c3718310c37694a3936905dfb