Directly propagate Activity titles to action bars

Window decor action bars already inherit window title changes if no
explicit action bar title has been set but from an Activity's
perspective setting the activity title is a stronger signal. This is
especially relevant when the activity's action bar has been provided
by the application in the form of a Toolbar.

Bug 17138048

Change-Id: Ibf042cab6bbda3dc29590c09d1a0910b6ba78870