Rewrite getentropy.

The error handling was a mess, resulting in an infinite loop
(and a test timeout) if you actually took the /dev/urandom
fallback. I'm also unconvinced that the getrandom(2) path was correct
because of the various reasons why we might get a short count back
(http://man7.org/linux/man-pages/man2/getrandom.2.html). In this version,
the two paths look very similar except for the getrandom(2) failure case,
where it's now much clearer that we just give up so that we can boot
(http://b/33059407).

Bug: http://b/33059407
Bug: http://b/67015565
Test: ran tests on hikey and a ICS Galaxy Nexus
Change-Id: Ie715d59b46f7a70bae66943c316168838787b40d