genrule: expand $$ to $$

An escaped $ sign in an expanded string needs to be re-escaped so it
can pass through to ninja.

Test: m -j libLLVMObject
Change-Id: I2f7890417fd6f534e6e21cace3d86ae9ef708ce5
diff --git a/android/expand.go b/android/expand.go
index 1013181..527c4ac 100644
--- a/android/expand.go
+++ b/android/expand.go
@@ -57,7 +57,7 @@
 		}
 		return "", len(s), fmt.Errorf("missing )")
 	case '$':
-		return s[0:1], 1, nil
+		return "$$", 1, nil
 	default:
 		i := strings.IndexFunc(s, unicode.IsSpace)
 		if i == 0 {