Switch from S to 31 for min_sdk_version.

Merged-In set from https://r.android.com/1875690, which does the
corresponding change in AOSP.

Test: vendor/google/build/mainline_modules_sdks.sh
Bug: 204364566
Ignore-AOSP-First: Fixed differently there: aosp/1875690
Change-Id: Ie64c87e23a12ae5ebe2b276829a3ab0a454a7a97
Merged-In: I93464af50e85cd59a8ba25b987520744e75ec0b3