[automerger skipped] Change the default background dexopt concurrency to 4. am: 7c107e133f -s ours

am skip reason: Merged-In Icd8aa89af0c11dd5f9cba5f0e0671376611385cc with SHA-1 350d7731a8 is already in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/art/+/26443633

Change-Id: Id6f19fc647a00061a53c1333f11b3c3c67f05c1e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>