Rename R8 library target and add R8 binary target. am: 414ff957ec am: b36be79579

Original change: https://android-review.googlesource.com/c/platform/prebuilts/r8/+/2053948

Change-Id: I34820b6a680e23e9a46b1fdb2c6f385b187bc77e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>