Fix Automated API Review issues.

1. Some API's argument/return value must be marked either
@NonNull or @Nullable.
2. Change some system APIs to public APIs.
3. Modify the method name

Bug: 126700123
Bug: 126702339
Bug: 126699682
Bug: 118296575
Bug: 126699216
Bug: 126699675
Bug: 126699429
Bug: 126699193
Bug: 123586045
Test: atest FrameworksNetTests
Change-Id: Iaa2832cdcf83758ed0fec81b954a0c63bc5a7bf6
diff --git a/services/core/java/com/android/server/ConnectivityService.java b/services/core/java/com/android/server/ConnectivityService.java
index 4416b4d..d818dc4 100644
--- a/services/core/java/com/android/server/ConnectivityService.java
+++ b/services/core/java/com/android/server/ConnectivityService.java
@@ -2174,7 +2174,7 @@
             if (VDBG) log("identical MTU - not setting");
             return;
         }
-        if (LinkProperties.isValidMtu(mtu, newLp.hasGlobalIPv6Address()) == false) {
+        if (!LinkProperties.isValidMtu(mtu, newLp.hasGlobalIpv6Address())) {
             if (mtu != 0) loge("Unexpected mtu value: " + mtu + ", " + iface);
             return;
         }