Jon Robson authored Jan 20, 2022 and Jdlrobson committed Mar 23, 2022

Can be merged when database rows have been updated to
no longer use skin version.

* Drop migration code
* Drop skin version preference
* Drop default skin version and existing accounts skin version
* Move skin definitions into skin.json
* Repurpose SkinVector as an abstract class
* Update READMEs

Bug: T301930
Bug: T294995
Bug: T302627
Change-Id: I7454d8f1cfdef81e7f3df476d8ce86736b46fff2