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

ui/status/build_progress_proto/build_progress.proto

@@ -0,0 +1,38 @@
+// Copyright 2020 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto2";
+
+package soong_build_progress;
+option go_package = "soong_build_progress_proto";
+
+message BuildProgress {
+  // Total number of actions in a build. The total actions will increase
+  // and might decrease during the course of a build.
+  optional uint64 total_actions = 1;
+
+  // Total number of completed build actions. This value will never decrease
+  // and finished_actions <= total_actions. At one point of the build, the
+  // finished_actions will be equal to total_actions. This may not represent
+  // that the build is completed as the total_actions may be increased for
+  // additional counted work or is doing non-counted work.
+  optional uint64 finished_actions = 2;
+
+  // Total number of current actions being executed during a course of a
+  // build and current_actions + finished_actions <= total_actions.
+  optional uint64 current_actions = 3;
+
+  // Total number of actions that reported as a failure.
+  optional uint64 failed_actions = 4;
+}