Always run adbd as root in recovery.

At present, invoking "adb shell" while in recovery results in:

- exec '/system/bin/sh' failed: No such file or directory (2) -

"adb shell" should invoke /sbin/sh, but cannot as /sbin and /sbin/recovery
lack other-user execute permission.  Invoking "adb root" to restart adbd as
root _does_ work, however this behavior may not be intuitive to users who
encounter the above error.

The solution implemented here is to always run adbd as root in recovery, so
it has permission to run /sbin/sh.  Furthemore, user shells in recovery are
not particularly useful and "su" doesn't exist, thus "adb root" is likely
to be invoked anyways.

Change-Id: Iaaa25090e85d970e9a076fef068f5fae8202ab0b