Unnecessary TextChanged callbacks are being called.

Unnecessary TextChanged callbacks are being called.
Change it to be called only once at the end.

example)

Typing "호두" on the keyboard
Whenever COMMIT/PREEDIT in OnInputMethodContextEvent(), TextChanged callback is called

 OnTextFieldTextChanged(159) >  [ㅎ]   // PRE_EDIT
 OnTextFieldTextChanged(159) >  [호]   // PRE_EDIT
 OnTextFieldTextChanged(159) >  [혿]   // PRE_EDIT
 OnTextFieldTextChanged(159) >  []     // PRE_EDIT
 OnTextFieldTextChanged(159) >  [호]   // COMMIT
 OnTextFieldTextChanged(159) >  [호두] // PRE_EDIT

If you change the text to call the TextChanged callback only once at the end of the change,
 OnTextFieldTextChanged(159) >  [ㅎ]
 OnTextFieldTextChanged(159) >  [호]
 OnTextFieldTextChanged(159) >  [혿]
 OnTextFieldTextChanged(159) >  [호두]

Change-Id: I039263bc156f0be161f37eeb1d0e6ecf39f9536c