Fix a leak in Parcel::writeBlob.

Was mistakenly assuming that Parcel::writeFileDescriptor took
ownership of the fd that was passed in.  It does not!
Added some comments and a default parameter to allow the caller
to specify whether it wishes the Parcel to take ownership.

Bug: 5563374
Change-Id: I5a12f51d582bf246ce90133cce7690bb9bca93f6