DSU service: Show which partition is being installed in notification

Instead of showing "Install in progress", show which partition is being
installed and the total number of partitions, for example:
"Install in progress: system partition [2/3]"

Bug: 169036106
Test: Start a DSU installation and observe the system notification
Change-Id: I81b414606b1e18109ade5a3480931b162bd2a01f