[Origin transition] Explicitly ensure the correct thread is being used.

The previous impl assumes the transition plays in main thread. But in
reality, it can run in any handler thread. This change fixes the
assumesption and ensure it always run in the correct thread.

Flag: EXEMPTED trivial fix
Bug: 347060315
Test: atest PlatformAnimationLibCoreTests
      manual test with test app
Change-Id: Ica8bb4f943e7b459a436765caee30bb43abca29c