Unshare mount namespace in bootchart's thread

When bootchart is enabled its thread shares the mount namespace context
with the main thread. This prevents the main thread to switch the mount
namespace later with setns().

So, unshare() the mount namespace of the bootchart thread.

Bug: 229983560
Test: rebooted with bootcharting on/off
    enter_default_mount_ns should succeeded.
Change-Id: Idac0d0efcb4f7f7d8a7cbcebf8fa2fa29f104c35
Merged-In: Idac0d0efcb4f7f7d8a7cbcebf8fa2fa29f104c35
(cherry picked from commit 7f8721b9)