Support NetworkStats#addEntries API

Currently, NetworkStats#addEntry creates a new instance
containing the requested result instead of mutating the
original object. This creates a heavy memory footprint
when the caller invokes it in a loop because a new NetworkStats
instance is created whenever an addEntry is called.

This change introduces a new API that allows callers to
add a set of entries to a NetworkStats instance. This new
API will clone the instance only once, return the newly
created (and modified) instance, and ensure that the original
instance remains immutable.

Additionally, the change adopts config libraries for flagging
the API and corresponding CTS tests, since Android U-based
branches are no longer downstream of current development branches.

Test: atest FrameworksNetTests:android.net.connectivity.android.net.netstats.NetworkStatsApiTest
Test: atest CtsNetTestCases:NetworkStatsApiTest
Bug: 335680025
Fix: 357772271
API-Coverage-Bug: 357772271
Change-Id: I58513844e681c0621a6107ec4b198cd3957f289d

framework/Android.bp

@@ -86,6 +86,7 @@ java_defaults {
         "framework-wifi.stubs.module_lib",
     ],
     static_libs: [
+        "com.android.net.flags-aconfig-java",
         // Not using the latest stable version because all functions in the latest version of
         // mdns_aidl_interface are deprecated.
         "mdns_aidl_interface-V1-java",