Disable UMS splash when phone in locked state

Currently devices which use UMS will display the UMS toggle screen
over the lockscreen. This would allow unauthorised users to access
the contents of the SD card, when plugged into USB, without first
unlocking the device.

Disable the UMS splash if the phone is in a locked state.

Change-Id: Ic18462758344c327e70e6a70d674f08d8ad51cb0