Move Animation Lib content to new "core" subpackage.

This is in preparation of the introduction of an outer layer of the
library for internal use only, with additional dependencies, which will
live in an "internal" subpackage.

I thought keeping the same package name would be better to avoid an LSC
and to keep the library consistent from the user's perspective.

NOTE: the only code changes are due to ktfmt.

Bug: 316352212
Flag: NA
Test: just a renaming, still builds

Change-Id: Ie1ee81ee453685e9bf26ee169a52e05e4d2a117e