Fix debug log for SpellCheckerSessionListenerImpl#TASK_CANCEL

This is a follow up CL for Ifd05565ac0c057c46ec88a3fb9094c04934041d.

SpellCheckerSessionListenerImpl.taskToString(TASK_CANCEL) has
somehow returned "STATE_WAIT_CONNECTION" probably because of
a copy-and-paste mistake.

With this CL, it now returns "TASK_CANCEL", which is observable
only in debug log anyway though.

Bug: 21319642
Change-Id: Ib0bf11494bd3f210e6e8192fc25a33c65f7d313d