Explain why unit tests need bootstrap bionic

The question of why the unit tests need the bootstrap bionic came up
while I was working on http://b/152572170.

Test: none
Change-Id: I08651200e2193c6ea4d3a734b59f022d69e5f154

tests/Android.bp

@@ -54,6 +54,9 @@ cc_defaults {
     sanitize: {
         address: false,
     },
+
+    // Use the bootstrap version of bionic because some tests call private APIs
+    // that aren't exposed by the APEX bionic stubs.
     bootstrap: true,
 
     product_variables: {