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

In branches that do not yet have a prebuilt module_31 SDK, this change
must be merged together with the next one reverting it to
module_current.

Bug: 189581241
Test: atest NetworkStackTests NetworkStackNextTests \
        CtsNetTestCasesLatestSdk

Merged-In: Ia55598123a5d4432464e9d1b1c637e9e0b58dd8e
Change-Id: Iac7779e17eab58a1ee5f5b4066cbfbf8c3ed59e5