Add mechanism for device-specific loop images

Bug: 144974129
Test: Manual - Testing OTA and Factory Reset
1) Tested with matching device name ({devicename}00000.png)
2) Tested with no matching device name (uses loop00000.png default)
3) Tested with empty string device name (uses loop00000.png default)

Change-Id: I4c73af82ab8826d1a43fe193a7616bc219d536e4