Make memunreachable_binder_test suitable for VTS

Link statically against libmemunreachable to avoid runtime
dependency on libmemunreachable.so.
Provide a better error when run as non-root.
Rename the test to MemunreachableBinderTest.
Add comments explaining the test sequence.

Bug: 28275695
Test: memunreachable_binder_test
Change-Id: Iddfba636205b7ca3ad31a6ba2c4e85abadd32d0d