Reduce persist threshold for lower warning/limit.

Default is 2MB persist threshold, but even that can be substantial
for devices on 100MB/month plans. This change gradually reduces the
persist threshold up to 8x lower (256kb outstanding) based on lowest
active policy.

Bug: 5382676
Change-Id: Ief4e8cdb169bfb151a3d1b45722a8eaa01926508

core/java/android/net/INetworkStatsService.aidl

@@ -42,5 +42,7 @@ interface INetworkStatsService {
     void setUidForeground(int uid, boolean uidForeground);
     /** Force update of statistics. */
     void forceUpdate();
+    /** Advise persistance threshold; may be overridden internally. */
+    void advisePersistThreshold(long thresholdBytes);
 
 }