Add android.net to proguard flags

Keep classes in android.net, as a followup change will have
some utility classes in that package in service-connectivity.

Ideally only "entry point" classes would be listed in the flags,
and the optimizer would figure out what is unreachable and
can be stripped out; but to be safe for now, list all classes.

Ignore-AOSP-First: Proguard rules not yet cherry-picked
Bug: 204830222
Change-Id: I45954f3bd23e84bacfd0f94875e10687e3e9cbc9
Test: m
diff --git a/service/proguard.flags b/service/proguard.flags
index 57086c7..4651088 100644
--- a/service/proguard.flags
+++ b/service/proguard.flags
@@ -1,4 +1,7 @@
 # Make sure proguard keeps all connectivity classes
+# TODO: instead of keeping everything, consider listing only "entry points"
+# (service loader, JNI registered methods, etc) and letting the optimizer do its job
+-keep class android.net.** { *; }
 -keep class com.android.connectivity.** { *; }
 -keep class com.android.net.** { *; }
 -keep class com.android.server.** { *; }