Implement AsRawFd trait for ParcelFileDescriptor

ParcelFileDescriptor can already be use as a reference of `File`, which
implements AsRawFd. Provide AsRawFd directly to make it easier for the
clients to use.

Bug: 194717985
Test: use in the client
Change-Id: I7a53367306b38cf0dd43364a698d0c83937d3e4d