Soong UI generates a proto file named build_progress.pb in $(OUT_DIR)
output directory that contains build action numbers (how many are executing,
finished and total) during the course of a build. This is for external
systems that invokes the Platform Build Systems and would like to know
the completion status.
Bug: b/150401146
Test: Wrote a bash script that continuously read the
build_progress.pb file and computed the build completed percentage
while building the aosp_arm-eng target. Compared the percentage between
the Soong output console and the one reported by the bash script.
Change-Id: I7c7347bc8e41958093892d8e2731c4f4169937dd
74b43998e7