[gwt-contrib] Change Activity to an abstract class, to allow its api to evolve. (issue1200801)

[rjrjr]

Reviewers: robertvawter,

Description:
Change Activity to an abstract class, to allow its api to evolve.
Gwt issue
http://code.google.com/p/google-web-toolkit/issues/detail?id=5730


Please review this at http://gwt-code-reviews.appspot.com/1200801/show

Affected files:
  M  
samples/expenses/src/main/java/com/google/gwt/sample/expenses/client/ExpenseReportDetails.java
  M  
samples/expenses/src/main/java/com/google/gwt/sample/expenses/client/ExpenseReportList.java
  M  
samples/expenses/src/main/java/com/google/gwt/sample/expenses/client/ExpensesActivityMapper.java
  D user/src/com/google/gwt/activity/shared/AbstractActivity.java
  M user/src/com/google/gwt/activity/shared/Activity.java
  M user/src/com/google/gwt/activity/shared/ActivityManager.java
  A user/src/com/google/gwt/activity/shared/IsActivity.java
  A user/src/com/google/gwt/activity/shared/SimpleActivity.java
  M user/test/com/google/gwt/activity/shared/ActivityManagerTest.java


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