Unshare mount namespace in bootchart's thread am: 1f85546be5 am: ad56552398

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

Change-Id: Icaa2ecde690f79966379401a11dc86d86bef29cb
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>