More than one finger at a time can trigger a system crash.

1. The crash was happening if: two active pointers are performing a drag;
   there are some inactive pointers down; the main dragging pointer (we are
   merging the dragging pointers into one) goes up; now an inactive pointer
   goes up and the explorer tries to inject up for the dragging pointer
   which is no longer in the event resulting in a crash. Basically two
   problems: inactive pointers were not ignored; 2) having only one
   active pointer should not only send the up event but also transition
   the explorer in touch exploring state.

bug:6874128

Change-Id: I341fc360ebc074fe3919d5ba3b98ee5cb08dd71e