Make class SetTimerSlackAction easier to maintain

Unify the Darwin and Linux class declarations. Move all #ifdefs for this
class into the .cpp file. This patch does not change any functionality.

Bug: 213617178
Test: Booted Android in Cuttlefish.
Change-Id: I6a3cedc75c2dbf8aa062edda76cae9959767a763
Signed-off-by: Bart Van Assche <bvanassche@google.com>
diff --git a/libprocessgroup/task_profiles.cpp b/libprocessgroup/task_profiles.cpp
index 74ba7f6..7fe984b 100644
--- a/libprocessgroup/task_profiles.cpp
+++ b/libprocessgroup/task_profiles.cpp
@@ -183,6 +183,12 @@
     return true;
 }
 
+#else
+
+bool SetTimerSlackAction::ExecuteForTask(int) const {
+    return true;
+};
+
 #endif
 
 bool SetAttributeAction::ExecuteForProcess(uid_t, pid_t pid) const {