rename generated asset manifest file back to `AssetManifest.bin` (from `AssetManifest.smcbin`) (#128529)

Closes https://github.com/flutter/flutter/issues/128456, which is now linked to in a code comment in this change.
Reopens https://github.com/flutter/flutter/issues/124883.

This effectively reverts https://github.com/flutter/flutter/pull/126077 and is intended to be cherry-picked into stable.