Allocate well-known tag for app store updates.

This way an app store can shift blame for update-related network
traffic onto the app that is being updated.

Using a well-known tag makes it easy for developers to identify
that they didn't explicitly request the traffic at runtime, similar
to how backup/restore traffic is handled.

Bug: 38282350
Test: builds, boots
Change-Id: I003dd7c9615d4ab318250f1e44fa5d195ac94d23