Add DomainVerificationSettings write/read to Settings

Introduces the concept of a domainSetId which represents an immutable
set of domains declared by the package. This is currently updated
whenever the package updates, hooked into the process for updating
a PackageSetting object.

Exempt-From-Owner-Approval: Already approved by owners on main branch

Bug: 163565712

Test: atest PackageManagerSettingsTests

Change-Id: I7c2c541cfdad7ddca7739424ee9e034fe616988b