OwnIdentity.OwnIdentity(): Use mInsertURIString

- Store the normalized FreenetURI to new intermediate local variable
before storing it to mInsertURIString because we need FreenetURI
functions
- Don't initialize mInsertURI anymore as it is deprecated and
initialized to null at its declaration.
OwnIdentity.OwnIdentity(): Add code quality TODO
OwnIdentity.getInsertURI(): Use mInsertURIString