Export ConfigurableCase and add constructors

Some soong code sets arch-variant properties in order to control a
module's default behavior. I'll make this continue to work, but long
term the arch-variant properties should be replaced with selects,
so expose an API for creating select statements in soong code.

Bug: 323382414
Test: m nothing --no-skip-soong-tests
Change-Id: I6c65d6e112b6f826f1027777b6fdf36915d34b1d
4 files changed