Introduce DexJavaLibraryIfItContainsClassFiles Buildable.

Summary:
`DexJavaLibraryIfItContainsClassFiles` is a `Buildable` that takes the output of a `JavaLibraryRule` and
and dexes it, assuming the output of the `JavaLibraryRule` contains `.class` files.
It relies on an `AccumulateClassNames` to determine whether there are `.class` files to dex.

Once `DexJavaLibraryIfItContainsClassFiles` has the path to the classes file,
it uses a `FileExistsAndIsNotEmptyStep` combined with a `ConditionalStep` to
make sure that the list of `.class` files is non-empty before running the `DxStep`
because `dx` will fail if it is run on input that does not contain any `.class` files.

Test Plan: Sandcastle builds.