Add code to enable USE_BAZEL=1 m functionality in AOSP.

Test: Manually, but builds currently fail due to a recent change to bazel_handler.go.
Change-Id: I9a45a2c73d1755032b11f6b913c03f83a28f63c5

cmd/soong_ui/main.go

@@ -470,7 +470,13 @@ func make(ctx build.Context, config build.Config, _ []string, logsDir string) {
 		ctx.Fatal("done")
 	}
 
-	toBuild := build.BuildAll
+	var toBuild int
+	if _, ok := config.Environment().Get("USE_BAZEL"); ok {
+		toBuild = build.BuildAllWithBazel
+	} else {
+		toBuild = build.BuildAll
+	}
+
 	if config.Checkbuild() {
 		toBuild |= build.RunBuildTests
 	}

ui/build/Android.bp

@@ -39,6 +39,7 @@ bootstrap_go_package {
         "blueprint-microfactory",
     ],
     srcs: [
+        "bazel.go",
         "build.go",
         "cleanbuild.go",
         "config.go",

ui/build/bazel.go

@@ -0,0 +1,54 @@
+// 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.
+
+package build
+
+import (
+	"path/filepath"
+	"strings"
+)
+
+func runBazel(ctx Context, config Config) {
+	// "droid" is the default ninja target.
+	outputGroups := "droid"
+	if len(config.ninjaArgs) > 0 {
+		// At this stage, the residue slice of args passed to ninja
+		// are the ninja targets to build, which can correspond directly
+		// to ninja_build's output_groups.
+		outputGroups = strings.Join(config.ninjaArgs, ",")
+	}
+
+	bazelExecutable := filepath.Join("tools", "bazel")
+	args := []string{
+		"build",
+		"--verbose_failures",
+		"--show_progress_rate_limit=0.05",
+		"--color=yes",
+		"--curses=yes",
+		"--show_timestamps",
+		"--announce_rc",
+		"--output_groups=" + outputGroups,
+		"//:" + config.TargetProduct() + "-" + config.TargetBuildVariant(),
+	}
+
+	cmd := Command(ctx, config, "bazel", bazelExecutable, args...)
+
+	cmd.Environment.Set("DIST_DIR", config.DistDir())
+	cmd.Environment.Set("SHELL", "/bin/bash")
+
+	ctx.Println(cmd.Cmd)
+	cmd.Dir = filepath.Join(config.OutDir(), "..")
+	ctx.Status.Status("Starting Bazel..")
+	cmd.RunAndStreamOrFatal()
+}

ui/build/build.go

@@ -84,8 +84,10 @@ const (
 	BuildSoong         = 1 << iota
 	BuildKati          = 1 << iota
 	BuildNinja         = 1 << iota
+	BuildBazel         = 1 << iota
 	RunBuildTests      = 1 << iota
 	BuildAll           = BuildProductConfig | BuildSoong | BuildKati | BuildNinja
+	BuildAllWithBazel  = BuildProductConfig | BuildSoong | BuildKati | BuildBazel
 )
 
 func checkProblematicFiles(ctx Context) {
@@ -257,6 +259,10 @@ func Build(ctx Context, config Config, what int) {
 		// Run ninja
 		runNinja(ctx, config)
 	}
+
+	if what&BuildBazel != 0 {
+		runBazel(ctx, config)
+	}
 }
 
 // distGzipFile writes a compressed copy of src to the distDir if dist is enabled.  Failures