Fix section exclusion POST body handling

Follows up Icbb7af323dcbfa3c517fbe9fb08e282b0bfdf382:

• actually read sections_to_exclude from POST body
• if not present, fall back to GET, so it doesn't have to be deployed exactly simultaneously with Ia88088da1b002c9d141e274653e35d594b786e95.

Bug: T308186
Change-Id: Ife6f98459fcee802cec8126044681b8a10f65389