MINOR: activity: allow to switch per-task lock/memory profiling at runtime

MINOR: activity: allow to switch per-task lock/memory profiling at runtime

Given that we already have "set profiling task", it's easy to permit to
enable/disable the lock and/or memory profiling at run time. However, the
change will only be applied next time the task profiling will be switched
from off/auto to on.

The patch is very minor and is best viewed with git show -b because it
indents a whole block that moves in a "if" clause.

This can be backported to 3.3 along with the two previous patches.