Show recents at correct time when starting activity in docked stack

Previous logic relied on the returnTo type of the task of the activity
we are launching which can get the wrong signal at times because the
original task might have been started from home, but since we are
already in docked mode it shouldn't cause recents activity to be
launched.
We now decide if recents ability should be shown based on if the home
stack is currently visible at the time we started the new activity.
Also, renamed ActivityStack.getStackVisibilityLocked() to
ActivityStack.shouldBeVisible() since it is used to determine if the
stack should be visible and also so it isn't confused with the new
method ActivityStack.isVisible() which returns true if the stack is
currently visible.

Test: manual
Change-Id: I051e72ce93c886d25526af2afef851c95812ab3e
Fixes: 37005549