Refactoring Activity Manager.

Mid-course corrections.

- Simplify ActivityStackSupervisor and ActivityStack constructors to
get passed parameters from ActivityManagerService.
- Store WindowManagerService referecne locally in
ActivityStackSupervisor and ActivityStack.
- Rename getTopStack to getFocusedStack in ActivityStackSupervisor
and ActivityManagerService.
- Move mWaitingActivityLaunched/reportActivityLaunchedLocked and
mWaitingActivityVisible/reportActivityVisibleLocked from ActivityStack
to ActivityStackSupervisor.
- Moved reportResumedActivity to ActivityStackSupervisor.
- Added a Handler to ActivityStackSupervisor. Will populate it on next
CL.

Change-Id: I1bbe5eb737c5cac6b896bc9748f329891e94d00f