Factor our staging step to own activity.

To make the Package Installer safe against acivity lifecyle I need to
make each installation step a separate activity. Also the activity's
with progress (staging, installation-progress) have to be carefully
crafted to work correctly with the activity lifecycle.

There will be further changed dealing with the other steps.

Test: Installed from content-URI
      Forced staging to fail and checked that dialog works
      Canceled staging via cancel button
Change-Id: I914fad6898f9ed9c71f18370811dca1986040561