Treat hidden API exemptions as whitelist.

This was a request from android-api-council, effectively to treat
exempted APIs as equivalent to public APIs. The reasoning for this is
that if the support library uses such APIs, we don't want to confuse
developers with a warning in the log.

To avoid examing the exemptions list on all light greylist API accesses,
also change the light greylist warn behavior to only print a warning in
the log if the app is debuggable. This means:
- less log spam from light greylist usages
- debuggable apps still get info about light greylist usage

Bug: 64382372
Test: m
Test: Boot device
Test: $ adb shell settings put global hidden_api_blacklist_exemptions \
Test:     "\"Landroid/app/Activity;->mWindow:,Landroid/app/Activity;->mCalled:\""
Test: Verified with test app & adb logcat
Change-Id: Ibada61b591517f7e72c7101aea04ff0ad4beb0ee
diff --git a/runtime/runtime.h b/runtime/runtime.h
index 03f17bc..c145937 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -1004,7 +1004,8 @@
   // Whether access checks on hidden API should be performed.
   hiddenapi::EnforcementPolicy hidden_api_policy_;
 
-  // List of signature prefixes of methods that have been removed from the blacklist
+  // List of signature prefixes of methods that have been removed from the blacklist, and treated
+  // as if whitelisted.
   std::vector<std::string> hidden_api_exemptions_;
 
   // Whether the application has used an API which is not restricted but we