Fix message names in dumpsys network_stack

Constants were stripped out by ProGuard, so
MessageUtils.findMessageNames would fail to find them.

Test: flashed, dumpsys network_stack shows message names
Fixes: 124619824
Change-Id: I285ce2ab52cdff48f17b422285a635bd34243498
diff --git a/proguard.flags b/proguard.flags
new file mode 100644
index 0000000..c60f6c3
--- /dev/null
+++ b/proguard.flags
@@ -0,0 +1,9 @@
+-keepclassmembers class android.net.ip.IpClient {
+    static final int CMD_*;
+    static final int EVENT_*;
+}
+
+-keepclassmembers class android.net.dhcp.DhcpClient {
+    static final int CMD_*;
+    static final int EVENT_*;
+}