API: Clean up redundant and ineffective usages of SystemApi and TestApi

Everything that is marked SystemApi or TestApi, but not @hide is still
part of the public SDK, it is therefore not sound to have that combination.

In the future, specifing such a combination will be considered an error
to prevent inadvertently exposing SystemApi and TestApi as public API.

Bug: 115333477
Change-Id: Ibd5d6a22862fdbc1e20a1cb3925280f5a682edea
Merged-In: Ibd5d6a22862fdbc1e20a1cb3925280f5a682edea
Test: METALAVA_PREPEND_ARGS="--error UnhiddenSystemApi" m checkapi
Exempt-From-Owner-Approval: API cleanup

core/java/android/net/TrafficStats.java

@@ -274,7 +274,6 @@ public class TrafficStats {
      * Changes only take effect during subsequent calls to
      * {@link #tagSocket(Socket)}.
      */
-    @SystemApi
     @SuppressLint("Doclava125")
     public static void setThreadStatsUid(int uid) {
         NetworkManagementSocketTagger.setThreadSocketStatsUid(uid);
@@ -313,7 +312,6 @@ public class TrafficStats {
      *
      * @see #setThreadStatsUid(int)
      */
-    @SystemApi
     @SuppressLint("Doclava125")
     public static void clearThreadStatsUid() {
         NetworkManagementSocketTagger.setThreadSocketStatsUid(-1);