cd to / before running soong_build .

This lets one avoid any decisions as to when to chdir there during its
execution and leads to better sandboxing because the pwd doesn't leak to
init() functions anymore.

Test: Manual.
Change-Id: I1560da8ed3a621249426f9e8908aa890c21e13ba
diff --git a/java/rro_test.go b/java/rro_test.go
index edbf170..061d9d3 100644
--- a/java/rro_test.go
+++ b/java/rro_test.go
@@ -20,6 +20,7 @@
 	"testing"
 
 	"android/soong/android"
+	"android/soong/shared"
 )
 
 func TestRuntimeResourceOverlay(t *testing.T) {
@@ -105,7 +106,7 @@
 
 	// Check device location.
 	path = androidMkEntries.EntryMap["LOCAL_MODULE_PATH"]
-	expectedPath = []string{"/tmp/target/product/test_device/product/overlay"}
+	expectedPath = []string{shared.JoinPath(buildDir, "../target/product/test_device/product/overlay")}
 	if !reflect.DeepEqual(path, expectedPath) {
 		t.Errorf("Unexpected LOCAL_MODULE_PATH value: %v, expected: %v", path, expectedPath)
 	}
@@ -114,7 +115,7 @@
 	m = ctx.ModuleForTests("foo_themed", "android_common")
 	androidMkEntries = android.AndroidMkEntriesForTest(t, ctx, m.Module())[0]
 	path = androidMkEntries.EntryMap["LOCAL_MODULE_PATH"]
-	expectedPath = []string{"/tmp/target/product/test_device/product/overlay/faza"}
+	expectedPath = []string{shared.JoinPath(buildDir, "../target/product/test_device/product/overlay/faza")}
 	if !reflect.DeepEqual(path, expectedPath) {
 		t.Errorf("Unexpected LOCAL_MODULE_PATH value: %v, expected: %v", path, expectedPath)
 	}
@@ -160,7 +161,7 @@
 
 	// Check device location.
 	path := android.AndroidMkEntriesForTest(t, ctx, m.Module())[0].EntryMap["LOCAL_MODULE_PATH"]
-	expectedPath := []string{"/tmp/target/product/test_device/product/overlay/default_theme"}
+	expectedPath := []string{shared.JoinPath(buildDir, "../target/product/test_device/product/overlay/default_theme")}
 	if !reflect.DeepEqual(path, expectedPath) {
 		t.Errorf("Unexpected LOCAL_MODULE_PATH value: %q, expected: %q", path, expectedPath)
 	}
@@ -179,7 +180,7 @@
 
 	// Check device location.
 	path = android.AndroidMkEntriesForTest(t, ctx, m.Module())[0].EntryMap["LOCAL_MODULE_PATH"]
-	expectedPath = []string{"/tmp/target/product/test_device/system/overlay"}
+	expectedPath = []string{shared.JoinPath(buildDir, "../target/product/test_device/system/overlay")}
 	if !reflect.DeepEqual(path, expectedPath) {
 		t.Errorf("Unexpected LOCAL_MODULE_PATH value: %v, expected: %v", path, expectedPath)
 	}