Hidden API blacklisting killswitch.

Just support "*" for now, meaning disable all API blacklisting for all
apps.

Test: Manually verified by:
- installing test app that accesses hidden API
- manually blacklist the API
- $ adb shell settings put global hidden_api_blacklist_exemptions \\*

This is a cherry pick of change I9a41a104742c9aaaf3a753e7b0f3a1106e37d4d3
from internal master.

Merged-In: I9a41a104742c9aaaf3a753e7b0f3a1106e37d4d3
Change-Id: Ibb2a1d054c1b099fc6093e37f24b2ed421a2313c
diff --git a/core/tests/coretests/src/android/provider/SettingsBackupTest.java b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
index ee276ef..757a70c 100644
--- a/core/tests/coretests/src/android/provider/SettingsBackupTest.java
+++ b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
@@ -413,7 +413,8 @@
                     Settings.Global.WTF_IS_FATAL,
                     Settings.Global.ZEN_MODE,
                     Settings.Global.ZEN_MODE_CONFIG_ETAG,
-                    Settings.Global.ZEN_MODE_RINGER_LEVEL);
+                    Settings.Global.ZEN_MODE_RINGER_LEVEL,
+                    Settings.Global.HIDDEN_API_BLACKLIST_EXEMPTIONS);
 
     private static final Set<String> BACKUP_BLACKLISTED_SECURE_SETTINGS =
              newHashSet(