Improved back button behavior in multi-window mode.

Pressing the back button on the last activity in a stack that isn't
full screen causes the home stack to move forward and hide all other
visible stacks. This change allows the affected stack to be removed
while keeping the other stack visible.

Bug: 19423645
Change-Id: I30d84f84d525c114ee4993c1e4178af70fe47440
2 files changed