libprocessgroup: Use uid_t consistently for UIDs

Found with -Wsign-conversion.

Test: m
Change-Id: I302ce3c2ffdd533fe45e98c5c7edbb442db46ae4
diff --git a/libprocessgroup/task_profiles.cpp b/libprocessgroup/task_profiles.cpp
index d5bd47c..eaafa5e 100644
--- a/libprocessgroup/task_profiles.cpp
+++ b/libprocessgroup/task_profiles.cpp
@@ -489,7 +489,7 @@
 }
 
 bool WriteFileAction::WriteValueToFile(const std::string& value_, ResourceCacheType cache_type,
-                                       int uid, int pid, bool logfailures) const {
+                                       uid_t uid, int pid, bool logfailures) const {
     std::string value(value_);
 
     value = StringReplace(value, "<uid>", std::to_string(uid), true);