Hide most symbols in libmemunreachable

In preparation for making libmemunreachble vendor_available, hide
everything but the public ABI using a version script.  The unit
tests now have to compile statically against libmemunreachable,
so split out the tests that run against the public API into a
separate test.

Bug: 132302484
Test: atest memunreachable_test memunreachable_unit_test
Change-Id: Ie9dec073fdeee342776566abedd50f7498476eca