Fix SOONG_DUMP_JSON_MODULE_GRAPH on a fresh checkout.

This CL adds generateJsonModuleGraph bazelBuildMode and ensures that it
returns Soong early, before Kati and Soong metrics collection begin
(which causes errors, since they're looking for files that Soong in json
dump mode did not write).

Test: TH
Test: rm -rf out && SOONG_DUMP_JSON_MODULE_GRAPH=/tmp/soong.json m nothing

Change-Id: I264eadb3b3b0cd6b6e7a65adc7b39bb1c01ca136
diff --git a/ui/build/soong.go b/ui/build/soong.go
index 19a47ae..a40457f 100644
--- a/ui/build/soong.go
+++ b/ui/build/soong.go
@@ -333,8 +333,9 @@
 }
 
 func shouldCollectBuildSoongMetrics(config Config) bool {
-	// Do not collect metrics protobuf if the soong_build binary ran as the bp2build converter.
-	return config.bazelBuildMode() != generateBuildFiles
+	// Do not collect metrics protobuf if the soong_build binary ran as the
+	// bp2build converter or the JSON graph dump.
+	return config.bazelBuildMode() != generateBuildFiles && config.bazelBuildMode() != generateJsonModuleGraph
 }
 
 func loadSoongBuildMetrics(ctx Context, config Config) *soong_metrics_proto.SoongBuildMetrics {