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

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

Change-Id: Ifa0c9977a67a06228282543aea5f31f2501fe990
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>