Add hidden ATM method to use in TaskFragmentOrganizer

getActivityBelow() is necessary to check if a split rule should
be applied for two activities on top of each other. This will
only report an activity that belongs to the same process.

When an application starts an activity B on top of an existing
activity A normally, there might be a split rule to put them
side-by-side. Organizer needs to know that activities A and B
are on top of each other to check, so it can either intercept
new activity launch or observe it after it happens and check the
rules then. The second approach results in a cleaner implementation,
but it also means that the organizer needs a reliable way to check
which activity is directly below the launched one. There is no
reliable source of info for that on the client side yet.

Bug: 190433398
Bug: 190433500
Test: Manual, using the reference implementation of the organizer.
Change-Id: I0e0116b1b0144e0ba4d19039d78c34495fbe4457