Delete staged file using separate activity

The InstallStaging activity was

1. copying the original file into the staged file
2. start the further install flow
3. wait for install to be over and then delete the staged file
   onActivityResult

It was hard to distinguish between the cases where the activity went
through a lifecycle and when the install was over as it was not
guaranteed that we receive onActivityResult before onResume (the
javadoc is confusing).

Hence finish the InstallStaging activity once staging is done and delete
file in a separate activity that has only this single purpose.

Now we rely on savedInstanceState == null to be only true for the very
first time this activity instance was started.

Change-Id: Icc093470151ed683224ddc0ee2665a9d32ab5687
Fixes: 67759288
Test: Installed app