Rename BuildDir and NinjaBuildDir.

These are just out/ and out/soong/ and the old names were quite
confusing.

Test: Presubmits.
Change-Id: I999ca62162062f27e449f83ffb00fa580d4065b8

java/app_test.go

@@ -1737,7 +1737,7 @@ func TestPackageNameOverride(t *testing.T) {
 
 			foo := result.ModuleForTests("foo", "android_common")
 
-			outSoongDir := result.Config.BuildDir()
+			outSoongDir := result.Config.SoongOutDir()
 
 			outputs := foo.AllOutputs()
 			outputMap := make(map[string]bool)