Make ActivityStack.mHistory private.

Isolate the Activity history for later conversion to Task-based
management.

Change-Id: I4b6bf22de035c768aa705df0cc4f84486e8ede56
diff --git a/services/java/com/android/server/am/PendingThumbnailsRecord.java b/services/java/com/android/server/am/PendingThumbnailsRecord.java
index ed478c9..c460791 100644
--- a/services/java/com/android/server/am/PendingThumbnailsRecord.java
+++ b/services/java/com/android/server/am/PendingThumbnailsRecord.java
@@ -27,13 +27,13 @@
 class PendingThumbnailsRecord
 {
     final IThumbnailReceiver receiver;   // who is waiting.
-    HashSet pendingRecords; // HistoryRecord objects we still wait for.
+    final HashSet<ActivityRecord> pendingRecords; // HistoryRecord objects we still wait for.
     boolean finished;       // Is pendingRecords empty?
 
     PendingThumbnailsRecord(IThumbnailReceiver _receiver)
     {
         receiver = _receiver;
-        pendingRecords = new HashSet();
+        pendingRecords = new HashSet<ActivityRecord>();
         finished = false;
     }
 }