This is generated with get_rust_pkg.py, plus
 - src/lib.rs, src/span_of.rs
 - patches/std.diff
   * Since Android does not support building with no-std, disable no-std
     and replace `core` with `std`.
 - Android.bp
 - patches/Android.bp.patch
   * remove doc-comment just to avoid one dependnecy. Can be added later.
 - TEST_MAPPING

Bug: 174797066
Test: mma
Test: tools/external_updater/updater.sh update --refresh --keep_date
      rust/crates/memoffset
Change-Id: I836d46eb71f1f0b1449318076e0eb266adbfe4c9