Make sure theme attributes don't get lost

When an app calls setTaskDescription it overrides the hidden
attributes derived from the theme. Make sure to not override these
if they aren't set in the new task description.

Test: Open Maps, go home, rotate, open maps again, make sure
navigation bar is black
Bug: 36703868

Change-Id: If3f8948fe81af324411c2e828834f5d81e3eeddd