Fix the run-as environment to better match su.

$USER and $LOGNAME along with $HOME were just plain wrong (leading to a
misleading interactive prompt), and it probably makes sense to reset the
variables that su would reset.

Bug: https://code.google.com/p/android/issues/detail?id=187438
Change-Id: I0404511453d371f36801f0212a8d72d93f0bc8ac