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
diff --git a/cmd/soong_ui/main.go b/cmd/soong_ui/main.go
index 774a872..a1b0c8e 100644
--- a/cmd/soong_ui/main.go
+++ b/cmd/soong_ui/main.go
@@ -470,7 +470,13 @@
 		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
 	}