ANDROID: cpu: send KOBJ_ONLINE event when enabling cpus

In case some sysfs nodes needs to be labeled with a different label than
sysfs then user needs to be notified when a core is brought back online.

Bug: 29359497
Bug: 120444461
Change-Id: I0395c86e01cd49c348fda8f93087d26f88557c91
Signed-off-by: Thierry Strudel <tstrudel@google.com>