Allow leading slash in path argument for addURI() method.

The original documentation for this class indicated a leading slash was possible
but the code did not support it.  Since then, the docs were changed to reflect
what the code does.

The purpose of this change is to allow the use of uri.getPath() as an argument.
With this change, the following can be done

matcher.addURI(ContactsContract.AUTHORITY,
        ContactsContract.CONTENT_FILTER_URI.getPath(), CONTACTS_FILTER)

instead of

matcher.addURI(ContactsContract.AUTHORITY,
        "contacts/filter", CONTACTS_FILTER)

Change-Id: I76a9e3133365be9fe7a8de86eae57f9eea1cd2a3