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/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index 86f4821..f4842c6 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -10387,6 +10387,14 @@
                 "storage_settings_clobber_threshold";
 
         /**
+         * Exemptions to the hidden API blacklist.
+         *
+         * @hide
+         */
+        public static final String HIDDEN_API_BLACKLIST_EXEMPTIONS =
+                "hidden_api_blacklist_exemptions";
+
+        /**
          * Settings to backup. This is here so that it's in the same place as the settings
          * keys and easy to update.
          *
diff --git a/core/proto/android/providers/settings.proto b/core/proto/android/providers/settings.proto
index ef6eb09..f78ebca 100644
--- a/core/proto/android/providers/settings.proto
+++ b/core/proto/android/providers/settings.proto
@@ -335,6 +335,7 @@
     SettingProto uninstalled_instant_app_min_cache_period = 290;
     SettingProto uninstalled_instant_app_max_cache_period = 291;
     SettingProto unused_static_shared_lib_min_cache_period = 292;
+    SettingProto hidden_api_blacklist_exemptions = 293;
 }
 
 message SecureSettingsProto {
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(