[flexiglass] Remove dependencies on SceneTestUtils.

There are some remaining Java tests that dependended on SceneTestUtils,
instead of converting them to Kotlin so they could use Kosmos directly,
I reduced SceneTestUtils to bare minimum, renamed it to
KosmosJavaAdapter, and moved it to the kosmos package.

Fix: 309503089
Test: N/A
Flag: N/A
Change-Id: I93cda19a5e4060424ebe1636f8e3d44f85294702