Bump NetworkStack to SDK 31

 - Set build SDK to module_31 instead of system_30 in release
   target
 - Set target SDK to 31 in release target instead of 30
 - Update version codes as per convention
 - Move "current" shim library to api31 shims
 - Add new "current" shim library for next development SDK

Ignore-AOSP-First: SDK 31 does not exist in AOSP. Will be partially
                   cherry-picked.

Bug: 189581241
Test: atest NetworkStackTests NetworkStackNextTests \
        CtsNetTestCasesLatestSdk

Change-Id: Ia55598123a5d4432464e9d1b1c637e9e0b58dd8e