Update D8 and R8 to 8.0.21-dev am: 593f56eef3

Original change: https://android-review.googlesource.com/c/platform/prebuilts/r8/+/2317098

Change-Id: Ida9a00f605a353947c947bcd892d60178c1b0920
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>