LT-15081: Problems Overwriting Writing Systems

Update and centralize method for seeking to end of XML node.
Prevent save of unmodified WS's on load (prevents
overwriting global WS before checking for updates).
Save only local LDML when importing from global (and keep
original timestamp from global).
Refactor the method that does most of the above.
Yet to test: how this interacts with Send/Receive.

Change-Id: I255a3959b141e8d5053bfc6d351cd8a43a274e13