Allow `first_version: "current"` in ndk_library.

Test: Added a test module that uses this
      find $SOONG_OUT -name libfoo.so # only shows up in current
Bug: None

Change-Id: Ic88d7c598cd5b6ded8fb851a7eaf859806336af7
diff --git a/cc/cc.go b/cc/cc.go
index b6e98b1..0071556 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -497,7 +497,7 @@
 		if err != nil {
 			ctx.PropertyErrorf("sdk_version", err.Error())
 		}
-		c.Properties.Sdk_version = strconv.Itoa(version)
+		c.Properties.Sdk_version = version
 	}
 }