Even more native input dispatch work in progress.

Added more tests.
Fixed a regression in Vector.
Fixed bugs in pointer tracking.
Fixed a starvation issue in PollLoop when setting or removing callbacks.
Fixed a couple of policy nits.

Modified the internal representation of MotionEvent to be more
efficient and more consistent.

Added code to skip/cancel virtual key processing when there are multiple
pointers down.  This helps to better disambiguate virtual key presses
from stray touches (such as cheek presses).

Change-Id: I2a7d2cce0195afb9125b23378baa94fd2fc6671c