Skip upgrade keys for adopted storage

Define a new scan flag to control whether or not we're performing
an initial scan for packages. On internal storage, an intial
scan occurs during every boot. For adopted storage, an intial scan
occurs immediately after the adopted volume is mounted or whenever
an application is moved.

Bug: 21815906
Change-Id: Ieda9668230597346bce9ee25ecfa204f931bcef6