Change the default background dexopt concurrency to 4. am: 350d7731a8 am: 11c8959243

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

Change-Id: I218b00f116773dc6f49438c6d092f9a95bb4f396
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>