Steady improvement to multi stack.

- Fix back button behavior with two stacks. Stopping activities were
held in that state indefinitely. This change causes IDLE_NOW_MSG to
be sent immediately for the last activity in a stack.

- Touch in non-focused stack was being ignored because of focus tests
in AbsListView.

- Change the focused stack when the activity focus changes. Renamed
mMainStack to mFocusedStack to reflect this.

- Resume all top stack activities when resuming.

- Assign intent task to ActivityRecord if it doesn't have a task.
Fixes bug 8433463.

Change-Id: I8d3c806234511697bc209ab99890730ffa514d20