Stop crash on repeated enters on virtual keyboard

Change https://googleplex-android-review.git.corp.google.com/#/c/742462/
removed the mDone since graying out the button seemed to remove the need
for this. However, you can also enter this command repeatedly from the
soft keyboard.

Put mDone back.

Bug: 23181625
Change-Id: I42e8ecb9824b17825778c9c113130506cc3a200b