Add --multitree-build flag

Add a flag that enables logic specific to multitree-build builds.

Bug: b/266730267
Test: treehugger, manual
Change-Id: Ia6f392525faa9d66bb6e0692b2a2e9c3b8493b0d
diff --git a/ui/build/soong.go b/ui/build/soong.go
index a5a3263..655ae35 100644
--- a/ui/build/soong.go
+++ b/ui/build/soong.go
@@ -168,6 +168,10 @@
 		commonArgs = append(commonArgs, "-t")
 	}
 
+	if !pb.config.multitreeBuild {
+		commonArgs = append(commonArgs, "--multitree-build")
+	}
+
 	commonArgs = append(commonArgs, "-l", filepath.Join(pb.config.FileListDir(), "Android.bp.list"))
 	invocationEnv := make(map[string]string)
 	if pb.debugPort != "" {
@@ -275,6 +279,9 @@
 	if len(config.bazelForceEnabledModules) > 0 {
 		mainSoongBuildExtraArgs = append(mainSoongBuildExtraArgs, "--bazel-force-enabled-modules="+config.bazelForceEnabledModules)
 	}
+	if config.MultitreeBuild() {
+		mainSoongBuildExtraArgs = append(mainSoongBuildExtraArgs, "--multitree-build")
+	}
 
 	queryviewDir := filepath.Join(config.SoongOutDir(), "queryview")
 	// The BUILD files will be generated in out/soong/.api_bp2build (no symlinks to src files)