run minadbd as shell user, remove unused code

Make minadbd drop its root privileges after initializing.  We need to
make the /tmp directory writable by the shell group so that it can
drop the sideloaded file there.
diff --git a/etc/init.rc b/etc/init.rc
index a301779..bf4ce62 100644
--- a/etc/init.rc
+++ b/etc/init.rc
@@ -15,6 +15,9 @@
     mkdir /cache
     mount /tmp /tmp tmpfs
 
+    chown root shell /tmp
+    chmod 0775 /tmp
+
     write /sys/class/android_usb/android0/enable 0
     write /sys/class/android_usb/android0/idVendor 18D1
     write /sys/class/android_usb/android0/idProduct D001