Revert ActivityManager changes for tasks. DO NOT MERGE

Keeping all activity=>task changes in master and removing them
from jb-mr2.

Revert "Update histories simultaneously."
Revert "Add null check to setAppGroupId."
Revert "Fix crashing bug in validator."
Revert "Switch topRunning* and moveTaskTo*"
Revert "Begin switch over to task based history."
Revert "Reset and reuse Iterators and don't new() one."
Revert "Remove AppWindowToken lists."
Revert "Fix build."
Revert "Remove unused App methods."
Revert "Stop using AppToken movement and start using Task."
Revert "Replace access to mAppTokens with AppTokenIterator"
Revert "Refactor setAppOpVisibility implementation."
Revert "Add AppWindowTokens to TaskList."
Revert "Make ActivityStack.mHistory private."
Revert "Migrate AppWindowToken lists into DisplayContent."

Change-Id: I5722c9a4956dccb52864207e2967690bc58e4ebb