Mark non-inplace Region operations WARN_UNUSED

Since the Java side equivalents operate in-place it can
be easy to assume Region.translate(x,y) will have some
effect whereas it is in fact a no-op unless the return
value is stored. Use WARN_UNUSED to help developers with
this.

Test: None
Change-Id: Idfd5b8b68dc144acaae62bd732eecdac7a745197