Explicitly reference framework-wifi app build rule

This works around a build-system limitation where it does not know
which jars to feed r8 to resolve all classes during proguarding.

Bug: 160453030
Bug: 169931783
Test: build_mainline_modules.sh (with prebuilt sdk)
Change-Id: I3d9f2049cf898b5e551a5c5764df423abaa5e92d

Tethering/Android.bp

@@ -100,6 +100,7 @@ java_defaults {
     ],
     libs: [
         "framework-tethering",
+        "framework-wifi",
     ],
     jarjar_rules: "jarjar-rules.txt",
     optimize: {