PackIndex: Make #getOffset protected to allow out-of-package subclasses

PackIndex: Make #getOffset protected to allow out-of-package subclasses

PackIndex#getOffset is abstract and package-private, limiting
subclasses to the same package. Given than the class and all other
relevant methods are public, this looks like an oversight and it
should be ok to write subclasses in other packages.

Make the method protected, allowing to write PackIndex subclasses in
other packages.

Change-Id: I7d3762b0a6c0681c29bd827bbaf094f1ee4583d2