adb: move adb to poll.

It is reported that the registered fd can be bigger than FD_SETSIZE, and can't be
handled by select(). By moving to poll(), we can remove the limitation.
Although we can't ignore the possibility that there is a fd leak, but we can
still make the potential bug more explicit by moving to poll().
We didn't move to epoll() because it is not supported on mac.

Bug: 23820751

Change-Id: Icb39329c4984f1fef749472c9e088682ee8c3444