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