Use upstream OpenBSD's arc4random.

The getentropy_linux.c is lightly modified to build on Android, but we're now
completely in sync with upstream OpenBSD's arc4random implementation.

(cherry picked from commit 2b67d7dee09852789d9ac7d8972ed6cdb2c18430)

Change-Id: Icc939b5fa2fcac3e15ff93735d2d34f67e9bb149
8 files changed