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
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 716378b..66d0c92 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -28,6 +28,13 @@
       "name": "memunreachable_test"
     },
     {
+      "name": "memunreachable_unit_test"
+    },
+    {
+      "name": "memunreachable_unit_test",
+      "host": true
+    },
+    {
       "name": "memunreachable_binder_test"
     },
     {