Correctly set the animation flag of the ActionBar

Showing/hiding the ActionBar is intentionally done without any animation
when done while the Activity is not visible (between onPostResume and
onStop). The issue is the animation flag was not set at all when the
ActionBar was initialized (i.e. getActionBar() called) between these two
lifecycle methods (because mActionBar was null).

A temporary solution is to call getActionBar() to initialize the
ActionBar in onCreate().

This patch simply keeps a flag of the current animation state and
update it whenever the ActionBar and/or the flag has been modified.