Update D8 and R8 to 8.1.3-dev am: 6b31f1c205

Original change: https://android-review.googlesource.com/c/platform/prebuilts/r8/+/2332142

Change-Id: I003b9f77e1357754c2cc2e63821c428eaa2c995d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>