All other AndroidX libraries have already increased their min SDK to
21.
This change renames private symbols to remove `V21` suffixes and
similar, but doesn't change public or protected symbols with similar
names, to avoid needless breakages/churn on users of the library.
Some of the dead code removal is more complex, so I've split it out
into follow-up changes to make it easier to review.
PiperOrigin-RevId: 651776556
6a9ff95bf0