onAttachedToWindowForTest

Along with onDetachedFromWindowForTest below, makes the LithoView think it's attached/ detached in a unit test environment. This also handles setting the same state for all LithoView children.

Implementation Note: Ideally, we'd just attach the LithoView to a View hierarchy and let AOSP handle all this for us. The reason we haven't is because attaching to an Activity while also trying to make sure the full LithoView is always considered visible (for the purposes of visibility events and incremental mount) proved difficult - if interested, see summary on the blame diff for this comment for more info.