Fixing a crash in the TouchExplorer.

1. If the runnable for performing a long press is not
   removed when all pointers are up and it is executed
   the explorer gets into delegating mode with no pointer
   down and the next down crashes the explorer. Added
   code to remove the long press runnable in a few places
   it was missing and also added a safety in the runnable
   to avoid executing it in case there are no active pointers.

bug:6557183

Change-Id: I9dab3de88fd08d8e2b38af18249ac551837c0736