Finish activity if uninstall is done

There are various ways how the Uninstaller activity signals back that
the uninstall is done. If the uninstall was started via
PackageInstaller.uninstall the signal comes back via the callback binder.
Once the binder is called, no furter UI is needed, hence we should call
finish().

Change-Id: Ib2f0b5e3e17634bae39db952542a33d7f7b17e5e
Bug: 64468607
Test: Used PackageInstall.uninstall API to uninstall and saw UI go away.