Always prefer poll over select.

Even with the fd leak fixed, we were still calling this code with fds
greater than 1024. From a tombstone from a dumpstate crash:

open files:
    fd 0: /dev/pts/0
...
    fd 18: /dev/stune/rt/tasks
    fd 1806: /data/anr/anr_2018-03-15-15-39-43-990
    fd 1807: /data/anr/anr_2018-03-15-15-39-43-991
    fd 1808: /data/anr/anr_2018-03-15-15-39-43-992
    fd 1809: /data/anr/anr_2018-03-15-15-39-43-993
    fd 1810: /data/anr/anr_2018-03-15-15-39-43-994
    fd 1811: /data/anr/anr_2018-03-15-15-39-43-995
    fd 1812: /data/anr/anr_2018-03-15-15-39-43-996
    fd 1813: /data/anr/anr_2018-03-15-15-39-43-997
    fd 1814: /data/anr/anr_2018-03-15-15-39-43-998
    fd 1815: /data/anr/anr_2018-03-15-15-39-43-999

Bug: http://b/73140330
Test: filled /data/anr/ with 2000 fake anrs, ran `dumpstate`

(cherry picked from commit ec616bc9)

Change-Id: Id3265e86fd987853e59261ae5f8e3bd03d618374