Switch from using epoll to poll.

The device python interpreter doesn't support epoll, and we only
really use it for waiting. This is a trivial change, because the
Python objects for poll and epoll support exactly the same
operations (except the timeout is specified in seconds or
milliseconds).

Bug: 36510518
Test: all_tests.sh passes on android-4.9
Test: resilient_rs_test.py passes on walleye
Change-Id: Icd45a790b7443f517c039c56def2df7b97bc0e0c