Add progress bars and make them show something

In the staging activity show a progress bar and make the layout of the
activity match the other activities (with a big center icon).

Update the progress bar while installing and show the same icon as used
by app stores (download icon).

Test: Only UI changes, hence I looked at the UIs.
Change-Id: I47eec806370eb0404851f5d7aee31f4626cd7af7