Add metadata files for plotters-backend. am: 3da56e3c5b am: 351eee5932 am: 28f19a9e7c am: 6fbe9993f9

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/plotters-backend/+/1621410

Change-Id: I9c997725178f21478bb9019c0b6826fd15bde34f