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