LinearLayoutManager

This is a copy of the LinearLayoutManager class and its helpers from AndroidX RecyclerView 1.2.1.

It was modified to change the anchor behavior when new items are added to the top of the list. See #6378.

Changes to this class should be limited to the functional changes we need. The aim is to make it easier to rebase on AndroidX's version of LinearLayoutManager. This means…

• we're not converting this class to Kotlin unless AndroidX changes their version,
• and we're ignoring warnings generated by unmodified code.