Fix TaskViewTouchController crash in seascape

Previously, we were using scroll direction POSITIVE as a catch all
to mean "up" but in seascape, we actually want NEGATIVE. Added
getUpDirection() to capture that. Tried to clarify the code a bit
by putting all the methods used solely by TaskViewTouchController
together with documentation. It's still pretty confusing and feels
redundant, but couldn't think of an obvious way to simplify.

Test: Swipe up and down on a task in all permutations of:
- 3 button mode
- Gesture navigation
- Portrait
- Landscape
- Seascape
- LTR
- RTL
- Home rotation allowed
- Home rotation disallowed

Fixes: 174009771
Fixes: 173567204
Change-Id: Id0f8d6f4365d888eb46182d8544d18206795dfb8