Query overlays using package and name

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