Convert libnativebridge[23]?-dummy to soong.

Bug: 37512442
Test: mmm system/core/libnativebridge/tests
Change-Id: Ib1ca9ef830f58041095226c2ba18d1302c7833b4
diff --git a/libnativebridge/Android.bp b/libnativebridge/Android.bp
index 5fb56f2..377b7dd 100644
--- a/libnativebridge/Android.bp
+++ b/libnativebridge/Android.bp
@@ -22,3 +22,5 @@
         },
     },
 }
+
+subdirs = ["tests"]
\ No newline at end of file