run-test build: Don't merge the shards by default. am: 720dcbc15b

Original change: https://android-review.googlesource.com/c/platform/art/+/2273466

Change-Id: I24ffbd6f525256461bc8f8fe63df5e48eac8c56e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>