Currently, there is no limtation for an app to request data usage callback, which is dangerous if the app fire hundreds of thousands requests and potientially this might cause OOM if the apps don't free them. Test: atest NetworkStatsObserversTest#testRegister_limit Bug: 229103088 Change-Id: I8299f46fd47a82ec9b25ba2e0d3c95db5512c331