Rename BuildDir and NinjaBuildDir.

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

Test: Presubmits.
Merged-In: I999ca62162062f27e449f83ffb00fa580d4065b8
Change-Id: I2fcd80a9e376a6842a06aa104a9f6f5c5e16f8c7

dexpreopt/config.go

@@ -159,7 +159,7 @@ func init() {
 }
 
 func constructPath(ctx android.PathContext, path string) android.Path {
-	buildDirPrefix := ctx.Config().BuildDir() + "/"
+	buildDirPrefix := ctx.Config().SoongOutDir() + "/"
 	if path == "" {
 		return nil
 	} else if strings.HasPrefix(path, buildDirPrefix) {