Merge "Make downcast_rs vendor available." am: 239b44a9c4 am: 5a95fe9758 am: c5aedd1730 am: a3388acfb1

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/downcast-rs/+/1886956

Change-Id: I933fb114e2705de695b5a395d5adda71feb1f9c4