FinishActivity: Do not write the currently set FORCE_SHOW_NAVBAR value

 * Check if the value for the setting is already applied and do not
   attempt to write it again. During the testing of SuW it was found out
   that there can be an edgecase where softkeys don't show up neither
   hardware keys work, even though the setting is applied (judging
   by the status of the KeyDisabler switch).

Change-Id: I902f26fe131e508974116f7607fd0b88921395d9