[SP18] Poll network stats in OffloadController to support data warning

The OEM implemented tether offload does not support
data warning since the HAL only tells the hardware about data limit
but not warning. However, to add such interface in HAL needs OEM to
comply and implement in hardware.

Thus, as a short-term solution, polls network statistics from HAL
and notify upper layer when it reaches the alert quota set by
NetworkStatsService.

Note that when CPU is sleeping, the data warning of tethering offload
will not work since the polling is also suspended.

Test: manual
Test: atest OffloadControllerTest
Bug: 149467454
Change-Id: I2467b64779b74cd5fec73b42fb303584f52cb1cb