Clean up activity/stack associations.

The stack currently holds a reference to resuming and pausing
activities. These are usually cleaned up when the activity ends or
the task is reparented. However, it is possible for an activity to
lose its reference to its task in other areas (such as
ActivityStarter), which can lead to the stack not being updated
correctly.

This changelist adds a method to the ActivityStack to disassociate
the stack from an ActivityRecord. In addition to places where this is
called when an activity ends, this method is invoked on the children
of a task when the task is reparented. The task member variable of
ActivityRecord is also now surrounded by a setter/getter, with the
setter always invoking the dissociation logic on a previous stack.

Test: bit FrameworksServicesTests:com.android.server.am.ActivityRecordTests
Change-Id: Iffeccdb6f011958896271673946acfed28856f53
Fixes: 36387417