TetherController.cpp: add O_CLOEXEC

Don't leak open file descriptors across execs to netd's children. This
can occur in the unlikely but theoretically possible event that one
thread is in writeToFile() and another thread happens to call exec().

Test: device boots with no obvious problems.
Change-Id: Iabd8eee46bf94d70894ca46e58484ccb8241513a