[multi-shade] Lock screen touch integration.

Integrates the multi-shade framework into the existing touch routing
system in NotificationShadeWindowView such that, on the lock screen,
swiping down shows the dual shades and tapping outside collapses them.

The approach taken was to implement a second interactor,
MultiShadeMotionEventInteractor and integrating it where the
DragDownHelper is currently integrated.

The next steps are:
a. Refactor the bouncer a bit so it can receive its expansion from
multi-shade, not just from the current shade expansion
b. Drive the expansion of the bouncer by dragging up when the shades are
collapsed, while on the lock screen
c. Figure out why clicking on the user switcher chip in the status bar
doesn't work in dual shade (there likely is some interference between
the right-hand side shade and the chip)

Bug: 274159734
Test: included new unit tests for MultiShadeMotionEventInteractor
Test: updated existing unit tests for MultiShadeInteractor
Test: manually verified the following interactions on the lock screen:
1. Dragging down anywhere reveals the correct shade
2. As the shades are revealed, the scrim fades in (there was a bug in
   this before)
3. When shade A is expanded, touching anywhere in the area of shade B
   collapses shade A
4. Clicking outside the shade collapses it
5. When any shade is expanded, it's not possible to touch things behind
   the scrim
6. When both shades are collapsed, touch passes correctly to the
   existing UI elements like the notifications

Change-Id: I94130e5e8dbb3b2a398452651855a47f231c2764