Refactor ActivityManagerService activity stack.

Introdude a new ActivityStack class that holds all of the
state and management of a stack of activities.  Paves the way
for having multiple activity stacks, though at this point
there should be no change in functionality and the activity
manager is still assuming there is only one stack.

Change-Id: Iea4859a24c9269061043755ec58a615028d4183b