Bump NetworkStack package version code

The version code in the manifest is only used for development
builds, as release builds dynamically replace it.

The first digits of the version code should represent the target SDK,
so set it to 31 for NetworkStackNext. To allow installing
development builds on any platform, also bump NetworkStack's
version to 309999999.

Bug: 170183552
Test: m

Change-Id: I55d95e0762df3357c1a8e58b8cb70865671a962a