Replace waitingVisible and other refactors

- ActivityRecord.waitingVisible is identical to
ActivityStackSupervisor.mWaitingVisibleActivities.contains(). This
ArrayList is never very large so much code can be simplified by
eliminating the waitingVisible member.

- The processStoppingActivityLocked() method can eliminate a lot of
variables by traversing the list top down. This makes the code
simpler to analyze and maintain.

- Declarations of ArrayLists do not need parameterization in the new
constructor. These have been removed in ActivityStackSupervisor.

Fixes item #5 of bug 18088522.

Change-Id: Ib9d648c5fa32c8dd7313882864886c929e1ebc21