Import RFC7421_PREFIX_LENGTH from NetworkStackConstants.

Remove the dependency on services-connectivity-shared-srcs
filegroup gradually.

Bug: 290885904
Test: TH
Change-Id: I6474563a3a0b521945e6467a2832862764ad4770
diff --git a/Tethering/src/android/net/ip/RouterAdvertisementDaemon.java b/Tethering/src/android/net/ip/RouterAdvertisementDaemon.java
index 18c2171..50d6c4b 100644
--- a/Tethering/src/android/net/ip/RouterAdvertisementDaemon.java
+++ b/Tethering/src/android/net/ip/RouterAdvertisementDaemon.java
@@ -16,7 +16,6 @@
 
 package android.net.ip;
 
-import static android.net.util.NetworkConstants.RFC7421_PREFIX_LENGTH;
 import static android.system.OsConstants.AF_INET6;
 import static android.system.OsConstants.IPPROTO_ICMPV6;
 import static android.system.OsConstants.SOCK_RAW;
@@ -30,6 +29,7 @@
 import static com.android.net.module.util.NetworkStackConstants.IPV6_MIN_MTU;
 import static com.android.net.module.util.NetworkStackConstants.PIO_FLAG_AUTONOMOUS;
 import static com.android.net.module.util.NetworkStackConstants.PIO_FLAG_ON_LINK;
+import static com.android.net.module.util.NetworkStackConstants.RFC7421_PREFIX_LENGTH;
 import static com.android.net.module.util.NetworkStackConstants.TAG_SYSTEM_NEIGHBOR;
 import static com.android.networkstack.tethering.util.TetheringUtils.getAllNodesForScopeId;