Define a proto file that generates the current build completion status.

Soong UI will generate a new proto file in the out directory for
other systems to read the build completion status during a course
of a build. The proto file contains build action counters where

total_actions: represents the total build actions of a build (will
increase and may decrease during a course of the build)

finished_actions: represents the number of executed build actions.
finished_actions never decreases and finished_actions <= total_actions.

current_actions: represents the number of build actions being executed
and current_actions + finished_actions <= total_actions.

Bug: 150401146
Test: executed regen.sh script.
Change-Id: I04f4c07855723d0684b141d88cb3529ab5d9fccd