Make ScopedFileDescriptor default ctor implicit

Having the default ctor marked explicit causes problems initializing
it using the {} syntax, which occurs, e.g. in
binder_parcel_utils.h:483

Without this change, compilation fails with:
"chosen constructor is explicit in copy-initialization"

Test: compiles
Change-Id: Ic261a3d9ce4bbd7d3d54402ccdcc4d4e02add271