Provide a easy way to access bpf maps from java

A wrapper for bpf map opening, reading/writing, and iteration.

Bug: 173167302
Test: atest BpfMapTest

Change-Id: I792b41978b322c9e4969cd7b6c35d6978ab86bc4
diff --git a/Tethering/proguard.flags b/Tethering/proguard.flags
index 86b9033..9ab56c2 100644
--- a/Tethering/proguard.flags
+++ b/Tethering/proguard.flags
@@ -4,6 +4,14 @@
     static final int EVENT_*;
 }
 
+-keep class com.android.networkstack.tethering.BpfMap {
+    native <methods>;
+}
+
+-keepclassmembers public class * extends com.android.networkstack.tethering.util.Struct {
+    public <init>(...);
+}
+
 -keepclassmembers class android.net.ip.IpServer {
     static final int CMD_*;
 }