Fabricated RROs identifiers are made using package name and overlay name
combinations. In the future, packages with multiple <overlay> tags will
use overlay name to distinguish between the tags.
This changes the OMS to manage overlays through OverlayIdentifiers.
The identifier is a overlay package and name combination. The name field
is optional so overlays without an overlay name can be managed.
This change simplifies the logic required to update the set of target
packages affected by overlay changes.
Bug: 172471315
Test: OverlayDeviceTests
Test: OverlayImpl tests
Change-Id: Iac679828d7480e5c7a2c1fe4ea8b73401d8d487f
5c8ba79373