[flexiglass] Removes support for multiple containers.

This was an premature optimization that we're deciding to remove for
now; greatly simplifies the Flexi-non-Flexi interaction.

Fix: 293005905
Test: Built and ran ComposeGallery
Test: Built and ran System UI without Compose
Test: Built and ran Sytem UI with Compose
Change-Id: I39cdcbecb936b64e55b40a04ea8ca897278fcf62
45 files changed