Extract OwnersData from Owners.

This change is intended to allow processing owners data during policy
version upgrade. Owners currently has a lot of side effects like
calls to other services, which doesn't allow it to be used as is.

This change extracts "pure" data object from Onwers, which just holds
the values and knows how to serialize/deserialize itself, which can
be used for upgrade.

Bug: 218639412
Test: atest com.android.server.devicepolicy
Change-Id: I7a79cafee4ff4437c55bb21d65adeb6815da2d0b