adb: fdevent: add fdevent_context_epoll.

Implement an epoll-backed fdevent_context, that reduces overhead when
we're polling many file descriptors. FdeventTest.smoke goes from ~5.2s
to ~3.3s when run on the host (after this patch's modification to change
it from chaining 10 file descriptors together to 512).

Test: adb_test on host
Test: adbd_test on blueline
Test: test_adb.py
Test: test_device.py
Change-Id: Iacf0093aa7bebea31e447c2cb012af72d8c3297e