MTP: remove usages of device identifiers with virtual serial number

Improving protection of user privacy
Use randomly generated virtual serial number that persists until factory reset

Bug: 117585389
Test: Open multiple MTP device
	Check MTP device property
	uninstall / re-install MTP driver
	WMP sync with multiple MTP device
	Enable MTP mode with multi-user
	Malformed MTP ID file

Change-Id: I7ed4ce81d9ca1e1c6605c59c73bcd2d26419a5d3