Fix type assertion error regarding dex_import

Ida6f7bb784efe74cc1fa0e8d370eaee803f08b0f made it possible to add
dex_import modules into apex, but that change had a bug. When creating
Android.mk for the dex_import module, the code executed an unchecked
type assertion to convert java.DexImport to java.Dependency, which
cannot be successful. This change fixes the bug by doing a checked type
assertion instead.

Bug: 157886942
Test: m (test added)
Change-Id: Id22c20d42effce539fab10b0d349bf340d467f02