Change the default background dexopt concurrency to 4. am: 350d7731a8

Original change: https://android-review.googlesource.com/c/platform/art/+/2984931

Change-Id: Idddb1e4c13228ae1fba6fadd2835856715baf6f5
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>