Don't time out when fetching tether offload stats.

Currently, fetching tethering offload stats has a 1000ms timeout,
after which we return stats of zero. However, returning zero is
invalid and will cause various parts of the network stats
accounting code to complain that statistics are moving backwards,
causing at least a log.wtf, and possibly a crash.

This CL removes the timeout entirely.

An alternative would have been to keep the timeout and return
null if the stats fetch failed. However, this complicates the
code, and if the HAL is persistently slow, could cause no stats
to be counted, ever. Given the impact of such behaviour on users'
data plans it is likely better to block until the stats are
collected.

Bug: 29337859
Bug: 32163131
Test: builds
Test: OffloadControllerTest passes
Change-Id: I9c9413d757f44e87a51ea7ca82b657cc6aa0d4ba