Replace ":strings" with "strings"

It looks like this is a typo and can cause parse errors with some xml
deserialization libraries.

Test: manual - set User Control Disabled Packages - restart device -
confirm that policy is still set
Bug: 291711966
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:4905c30a74d2037aef7e8e91912b08c4897e7dd5)
Merged-In: Ib8eb2356ece3d9f0e85b7b8c455e58106a1d84f1
Change-Id: Ib8eb2356ece3d9f0e85b7b8c455e58106a1d84f1
diff --git a/services/devicepolicy/java/com/android/server/devicepolicy/StringSetPolicySerializer.java b/services/devicepolicy/java/com/android/server/devicepolicy/StringSetPolicySerializer.java
index 24d0521..0265453e 100644
--- a/services/devicepolicy/java/com/android/server/devicepolicy/StringSetPolicySerializer.java
+++ b/services/devicepolicy/java/com/android/server/devicepolicy/StringSetPolicySerializer.java
@@ -32,9 +32,8 @@
 
 // TODO(scottjonathan): Replace with generic set implementation
 final class StringSetPolicySerializer extends PolicySerializer<Set<String>> {
-    private static final String ATTR_VALUES = ":strings";
+    private static final String ATTR_VALUES = "strings";
     private static final String ATTR_VALUES_SEPARATOR = ";";
-
     @Override
     void saveToXml(PolicyKey policyKey, TypedXmlSerializer serializer,
             @NonNull Set<String> value) throws IOException {