Fixed SpellCheckerSessions leak in TextView

Bug 5709546

Change 1a22db29 created a potential leak: setText calls
sendOnTextChange that now calls updateSpellCheckSpans.

The original call to updateSpellCheckSpans made sure the
View had been attached to a window, through the existence
of the SpellChecker object.

The mSpellChecker object will now only be created when the
View is attached, all other spell related method will do
nothing if it has not been initialized.

Change-Id: I8885c12be6395e2346c0ed057bb6a275256f012b