DSUService: stopSelf() only if no ongoing installation

If you are fast and start a DSU installation before the BOOT_COMPLETE
broadcast receiver can start, than the executeNotifyIfInUseCommand()
would destroy the on-going installation session, which is mildly
irritating.

Bug: 165471299
Test: Presubmit
Change-Id: Iaf6a165ece7d2b4a531a78845122046c19d456a7