Avoid killing the FUSE daemon during unmount

The FUSE daemon is often holding fds on behalf of other apps and if a
volume is ejected the daemon would often get killed first while vold
is walking /proc/<pid>/fd to kill pids with open fds on the
volume. This is required for the volume unmount successfully.

To mitigate this, we avoid killing the FUSE daemon during the usual
/proc walk. This ensures that we first send SIGINT, SIGTERM and
SIGKILL to other apps first. There is an additional SIGKILL attempt
and on that last attempt, we kill the FUSE daemon as a last resort

Test: Manual
Bug: 171673908
Change-Id: I100d2ce4cb4c145cbb49e0696842e97dfba2c1c9