Ignore bugreportz output when it's not supported.

On devices running M or below, calling 'bugreportz -v' writes
'/system/bin/sh: bugreportz: not found' in the stdout output, which must
be redirected to stderr so it's not shown in the flat-file bugreport,
above the bugreport header.

BUG: 30451114

Change-Id: I942c92fdf6ae85e0cde7b9f94b9eb0b1fecad77a