Add genrule for the current sdk proto

This is intended to be added to all apexes exposing SDK apis. The
version can then be bumped by changing the value in a single place.
The current version is 0.

Also build this into the sdkext module.

Bug: 137191822
Test: m com.android.sdkext
Change-Id: Ic2cd92ad1d184c8a01f0bc3d4fc21deb588cb7e8