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 \\*

Change-Id: I9a41a104742c9aaaf3a753e7b0f3a1106e37d4d3
diff --git a/core/tests/coretests/src/android/provider/SettingsBackupTest.java b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
index c36026c..dc8ed9e 100644
--- a/core/tests/coretests/src/android/provider/SettingsBackupTest.java
+++ b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
@@ -452,7 +452,8 @@
                     Settings.Global.ZEN_MODE_RINGER_LEVEL,
                     Settings.Global.ZRAM_ENABLED,
                     Settings.Global.OVERRIDE_SETTINGS_PROVIDER_RESTORE_ANY_VERSION,
-                    Settings.Global.CHAINED_BATTERY_ATTRIBUTION_ENABLED);
+                    Settings.Global.CHAINED_BATTERY_ATTRIBUTION_ENABLED,
+                    Settings.Global.HIDDEN_API_BLACKLIST_EXEMPTIONS);
 
     private static final Set<String> BACKUP_BLACKLISTED_SECURE_SETTINGS =
              newHashSet(