Declare android.hardware.radio@1.6 as system_ext module

Minor versions of android.hardware.radio were all declared but version 1.6

Bug: 207067211
Change-Id: Ib77a9b6daa2e70983e9a37716f3f7b059ec1f97f
diff --git a/radio/1.6/Android.bp b/radio/1.6/Android.bp
index 4b79c0a..2137e8d 100644
--- a/radio/1.6/Android.bp
+++ b/radio/1.6/Android.bp
@@ -29,4 +29,5 @@
         "android.hidl.safe_union@1.0",
     ],
     gen_java: true,
+    system_ext_specific: true,
 }