Introduce YangInstanceIdentifier.getAncestor()

Introduce YangInstanceIdentifier.getAncestor()

We have callsites which want to create a truncated
YangInstanceIdentifier and open-code instantiation. This is not
efficient, as those users do not have an understanding of the
implementation details, thus they have to resort to Iterables.limit()
and similar -- which forces copying of path arguments and untrusted
instantiation of YangInstanceIdentifier.

This patch introduces getAncestor(int), which performs this operation
with explicit knowledge of a particular implementation, resulting in
as much reuse as possible: for FixedYangInstanceIdentifier it uses a
subList(), for StackedYangInstanceIdentifier we return either a
direct/indirect parent, or use FixedYangInstanceIdentifier's version.

Change-Id: I355200199ac9791bb6d027fcdfa2a1162208a627
Signed-off-by: Robert Varga <rovarga@cisco.com>