Combines Share, promotes to Action Bar

Previously, only the reset action appeared on the Action Bar. Given the additional work that has gone into Export Contractions (more to be done as well), the Share All is less useful now. Therefore we can raise the prominance of the single share action to the Action Bar.

Due to space constraints, only show the reset button when screen width>450dp (i.e., normal phones in landscape, tablets).

Reset button also becomes disabled if there are no contractions and enable status is now updated when the underlying data changes.