Change qcom-usb-sh from core class to main class

On an encrypted device, qcom-usb-sh sets persist property
on early boot when /data is at tmpfs.So this persist
property write is lost when /data will be at dm-0
afterwards during encryption process.So, changing to main
class ensures to properly set persistproperty after /data
is at dm-0 as main class will be started
after encryption setup.

CRs-Fixed: 778812

Change-Id: I3d8398d51ef1eab66f76ab99492ea2b4507d3853

rootdir/etc/init.qcom.usb.rc

@@ -41,7 +41,7 @@ on fs
     mkdir /dev/bus/usb 0755 root root
 
 service qcom-usb-sh /system/bin/sh /init.qcom.usb.sh
-    class core
+    class main
     user root
     oneshot