Add flag for MediaRouter2-based InfoMediaManager

InfoMediaManager.createInstance() will gate MediaRouter2-based and
MediaRouter2Manager-based instances based on the new flag.

Also, return a NoOpInfoMediaManager when the target package name does
not exist.

Bug: 192657812
Test: Testmapping + Manual Flag Testing
Change-Id: I8dcdd1c1366ee4246b8a4805f2347a810f51afec
diff --git a/packages/SettingsLib/Android.bp b/packages/SettingsLib/Android.bp
index c244ca0..3d35bad 100644
--- a/packages/SettingsLib/Android.bp
+++ b/packages/SettingsLib/Android.bp
@@ -58,6 +58,7 @@
         "setupdesign",
         "zxing-core-1.7",
         "androidx.room_room-runtime",
+        "settingslib_flags_lib",
 
     ],
 
@@ -88,3 +89,16 @@
         "SettingsLib",
     ],
 }
+
+aconfig_declarations {
+    name: "settingslib_media_flags",
+    package: "com.android.settingslib.media.flags",
+    srcs: [
+        "aconfig/settingslib_media_flag_declarations.aconfig",
+    ],
+}
+
+java_aconfig_library {
+    name: "settingslib_flags_lib",
+    aconfig_declarations: "settingslib_media_flags",
+}