Use final in favor of initialization safety.

Making a member field final would be beneficial not
only for the readability but also for the initialization
safety.
Leaving SpellCheckerSession#mSpellCheckerSessionListener
non-final does not make sense not only because we never
change that member field once SpellCheckerSession object
is created and but also because SpellCheckerSession
instance is designed to be accessed from multiple threads
at the same time, no matter if it has something to do
with Bug 18945456 or not.

Change-Id: I1a7ebb54a5d0beddee8799fc5b0800c6e1059099