Fix crash with opening tabs in background

Fixed issue where timestamp was not set for background tabs. This
fixed issue where MemoryMonitor was getting null timestamp for tabs.
Also avoid purging active tab in MemoryMonitor if its foreground.

Change-Id: Ib79e8667ab8aa227881ca0f413ed51a8d6da8afa
diff --git a/src/com/android/browser/Tab.java b/src/com/android/browser/Tab.java
index ef9f468..786e376 100644
--- a/src/com/android/browser/Tab.java
+++ b/src/com/android/browser/Tab.java
@@ -1223,6 +1223,7 @@
         mDataController = DataController.getInstance(mContext);
         mCurrentState = new PageState(mContext, w != null
                 ? w.isPrivateBrowsingEnabled() : false);
+        setTimeStamp();
         mInPageLoad = false;
         mInForeground = false;