create a new top-level makefile target "clean.internal"

so I can clean just the non-external stuff when re-running my
clang plugins

Change-Id: I251ceba7e57454a91da5b1082192bb2d211e5406
use SimpleReferenceObject in xmloff module

to replace hand-rolled version

Change-Id: I36d4ce14d0c0c8a4b224a240c6e25c8df99b78aa
use SimpleReferenceObject in accessibility

to replace hand-rolled version

Change-Id: I9dd8d02024702972722b43f2654db91eb3da3794