Tell the system when tethering offload hits a limit.

Add a new tetherLimitReached method to INetworkManagementService,
and call it when the HAL notifies OffloadController because the
limit has been reached.

Bug: 29337859
Bug: 32163131
Test: builds
Test: OffloadControllerTest passes

(cherry picked from commit d66cf56b)

Change-Id: I89719fe7ec8bfd3c85d6cdca9c0d449aea86ef9d
Merged-In: I026e6aa9e7b371f316c0d97c3cf5e78abc1f5263