Unshare mount namespace in bootchart's thread am: 1f85546be5

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/core/+/17994482

Change-Id: I343cddbb8c78b44600e611c73fcb98afc0d0e093
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>