Add build progress in Soong UI

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
diff --git a/cmd/soong_ui/main.go b/cmd/soong_ui/main.go
index db61fba..1d94f02 100644
--- a/cmd/soong_ui/main.go
+++ b/cmd/soong_ui/main.go
@@ -173,6 +173,7 @@
 	stat.AddOutput(status.NewErrorLog(log, filepath.Join(logsDir, c.logsPrefix+"error.log")))
 	stat.AddOutput(status.NewProtoErrorLog(log, filepath.Join(logsDir, c.logsPrefix+"build_error")))
 	stat.AddOutput(status.NewCriticalPath(log))
+	stat.AddOutput(status.NewBuildProgressLog(log, filepath.Join(logsDir, c.logsPrefix+"build_progress.pb")))
 
 	buildCtx.Verbosef("Detected %.3v GB total RAM", float32(config.TotalRAM())/(1024*1024*1024))
 	buildCtx.Verbosef("Parallelism (local/remote/highmem): %v/%v/%v",