Use SuffixArrayIndexInterface opaque type instead of the underlying data pointer.

bsdiff interface is changing such that it hides the suffix array
pointer from the public interface. This allows to use a different
suffix array data size depending on the input size, running much faster
in the normal case.

Bug: 34220646
Test: `make checkbuild`; Ran an incremental update generation on a non-A/B device.
Change-Id: I78e766da56cf28bc7774b8c8e58527bc11d919fb