Move Tether4Key and Tether4Value to common libs

EthernetTetheringTest needs to parse BPF map content for IPv4
forwarding rule tests. Move the struct class files to common
libs.

Compare Tether4{Key, Value}.java between frameworks/libs/net/common
and packages/modules/Connectivity/Tethering.
There is only the package name change as expected.

$ diff packages/modules/Connectivity/Tethering/src/com/android/networkstack/tethering/Tether4Key.java frameworks/libs/net/common/device/com/android/net/module/util/bpf/Tether4Key.java
17c17
< package com.android.networkstack.tethering;
---
> package com.android.net.module.util.bpf;

$ diff packages/modules/Connectivity/Tethering/src/com/android/networkstack/tethering/Tether4Value.java frameworks/libs/net/common/device/com/android/net/module/util/bpf/Tether4Value.java
17c17
< package com.android.networkstack.tethering;
---
> package com.android.net.module.util.bpf;

Test: atest TetheringCoverageTests

Change-Id: I77142c784e20fd74d4aa5dd34858159e232abbe3
diff --git a/staticlibs/Android.bp b/staticlibs/Android.bp
index 047d51e..513b201 100644
--- a/staticlibs/Android.bp
+++ b/staticlibs/Android.bp
@@ -116,6 +116,7 @@
         "device/com/android/net/module/util/JniUtil.java",
         "device/com/android/net/module/util/Struct.java",
         "device/com/android/net/module/util/TcUtils.java",
+        "device/com/android/net/module/util/bpf/*.java",
     ],
     sdk_version: "module_current",
     min_sdk_version: "29",