Consolidate resume activity code around focused stack.

Cleaned up the code to make it obvious that only the top
activity in the focused stack should be in the resumes state.

Bug: 26273032
Change-Id: I8d60270f707fe022007c59d25f5678b33a005acf
diff --git a/services/core/java/com/android/server/am/UserController.java b/services/core/java/com/android/server/am/UserController.java
index 20b1e60..4cdf6b8 100644
--- a/services/core/java/com/android/server/am/UserController.java
+++ b/services/core/java/com/android/server/am/UserController.java
@@ -931,7 +931,7 @@
         if (homeInFront) {
             mService.startHomeActivityLocked(newUserId, "moveUserToForeground");
         } else {
-            mService.mStackSupervisor.resumeTopActivitiesLocked();
+            mService.mStackSupervisor.resumeFocusedStackTopActivityLocked();
         }
         EventLogTags.writeAmSwitchUser(newUserId);
         getUserManager().onUserForeground(newUserId);