Make active_concurrent_copying_collector_ atomic am: 45aa2affa5 am: 0ad7d8776d

Original change: https://android-review.googlesource.com/c/platform/art/+/1520678

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I254fe310199f35b5297d2c15aebd48eea89e1848