binder_rs: Add null parcelable flags

This adds NON_NULL_PARCELABLE_FLAG and NULL_PARCELABLE_FLAG
to parcel.rs as equivalents to Parcel::kNonNullParcelableFlag
and Parcel::kNullParcelableFlag from C++, respectively.

Bug: 169035750
Test: m
Change-Id: If9aab541bb8ff7fec14bcd590af074f564712c23
2 files changed