Stop splitting for mouse events.

Events from multi-touch touchpads have mouse source and can have
multiple pointers. This is a whole gesture and shouldn't be split into
multiple views.

Bug: 140312476
Test: Attached a touchpad to crosshatch (in my case I used the touch pad
on DualShock 4). Moved the cursor around the edge of two scrollable
views and perform 2-finger scroll. Only one of the view responds to the
scroll. Note it may not be the view under the cursor that responds to
it, which is another bug that needs to be fixed.
Test: Multi-finger touches on touchscreen can still be split.

Change-Id: I1c1250291fa8a88b33f84ca3f80c8b7ac3a61e48