Add /dev/stdin, /dev/stdout, and /dev/stderr.

Bug: http://b/31824379
Test: `adb shell ls -l /dev/std*`
Change-Id: I6af7ff205e12c10e958be263c3f1c429d48c0bbc

rootdir/init.rc

@@ -49,6 +49,10 @@ on init
     copy /proc/cmdline /dev/urandom
     copy /default.prop /dev/urandom
 
+    symlink /proc/self/fd/0 /dev/stdin
+    symlink /proc/self/fd/1 /dev/stdout
+    symlink /proc/self/fd/2 /dev/stderr
+
     symlink /system/bin /bin
     symlink /system/etc /etc