Fix propagating EMPTY_NINJA_FILE from multiproduct_kati to minibp

I6dca478f356f56a8aee1e457d71439272351390b replaced calling
build/blueprint/bootstrap.bash with running minibp directly,
but didn't propagate the EMPTY_NINJA_FILE environment variable
to minibp.  Since everything that uses EMPTY_NINJA_FILE is
executed directly from soong_ui now, replace the EMPTY_NINJA_FILE
environment variable with a config bool and pass it to minibp.

Bug: 189148777
Test: build/soong/build_test.bash --dist --incremental --shard-count=300 --shard=1 && du out/multiproduct
Change-Id: I4d64275ce02c5d68948012f71ac4dc3795af9e85
diff --git a/ui/build/config.go b/ui/build/config.go
index 1d1f71f..3ebde0d 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -72,6 +72,9 @@
 	// During Bazel execution, Bazel cannot write outside OUT_DIR.
 	// So if DIST_DIR is set to an external dir (outside of OUT_DIR), we need to rig it temporarily and then migrate files at the end of the build.
 	riggedDistDirForBazel string
+
+	// Set by multiproduct_kati
+	emptyNinjaFile bool
 }
 
 const srcDirFileCheck = "build/soong/root.bp"
@@ -203,9 +206,6 @@
 		"ANDROID_DEV_SCRIPTS",
 		"ANDROID_EMULATOR_PREBUILTS",
 		"ANDROID_PRE_BUILD_PATHS",
-
-		// Only set in multiproduct_kati after config generation
-		"EMPTY_NINJA_FILE",
 	)
 
 	if ret.UseGoma() || ret.ForceUseGoma() {
@@ -1189,3 +1189,11 @@
 func (c *configImpl) BazelMetricsDir() string {
 	return filepath.Join(c.LogsDir(), "bazel_metrics")
 }
+
+func (c *configImpl) SetEmptyNinjaFile(v bool) {
+	c.emptyNinjaFile = v
+}
+
+func (c *configImpl) EmptyNinjaFile() bool {
+	return c.emptyNinjaFile
+}