1. While a default target is very convenient, having two targets for the
widget on a phone takes too much space and the apps do not have room
to add other essential affordances to the action bar. Making the default
activity show only of the screen is large enough - the action bar
accommodates at least four targets i.e. 500dip.
2. Fixed a bug where changing the backing model of an ActivityChooserView
when the view is not shown did not remove the observer for the old
model, hence the View can get into a state where it responds to changes
of two models while presenting only one. Also in such a case the view
would leak since the singleton model will keep a handle to it.
3. Updated the documentation of share action provider to explain how a
a developer can change the backing history file and refresh the UI
based on the historical data in that file.
bug:6447692
Change-Id: Id5c9e54cd5df322ded8574ba028cb680e628243b
340e2611de