Changed uid output parameter from an int array to a list of strings.

Why?: 1) Returning an array list is unsafe because it must be allocated in Java and C++ must not change the size. 2) List<Integer> is not supported by AIDL, but List<String> is. I decided it was simpler to pass back integers encoded as strings than to create yet another parcelable.

Bug: b/119616956
Test: ./list_auth_bound_keys_test.sh
Test: Temporarily modified settings app to call listUidsOfAuthBoundKeys
Change-Id: I3bf7578c96e800c8d35fba897f52220136dcd657