WifiManager: Add callback for traffic poller

The current mechanism for traffic poller directly exposes the wifi
service's internal message handler to privileged apps. Move away from
this mechanism and expose an API to let apps register for callbacks from
traffic poller. This API uses binder IPC for registration and
invocation of callbacks. Clients are automatically removed on binder
death.
This will also let us mark WifiManager#getWifiMessenger() method
private to prevent any apps from getting access to this internal messenger.

Note: This is based on the existing SoftapCallback registration
mechanism.

Also, fixed a bunch of checkstyle errors that were not caught
previously.

Bug: 27074039
Test: Unit tests
Test: Verified the data indicators on Sysui
Change-Id: I505bd2398552be99ccca7b7d3c31a488aa72af70